High-quality silicon single crystal and method of producing the same

ABSTRACT

A method of producing a high-quality silicon single crystal of a large diameter and a long size in a good yield by controlling the positions where ring-like oxygen-induced stacking faults (R-OSF) occur in the crystal faces and minimizing grown-in defects such a dislocation clusters and infrared scattering bodies that are introduced in the pulling step. Wafers produced from the above-high-quality silicon single crystal contain little harmful defects that would deteriorate device characteristics and can be effectively adapted to larger scale integration and size reduction of the devices. Therefore, the method can be extensively utilized in the field of producing semiconductor silicon single crystals.

TECHNICAL FIELD

This invention relates to silicon single crystals for use assemiconducting materials. More particularly, it is directed to ahigh-quality silicon single crystal that is grown by a Czochralskimethod (hereinafter referred to as “CZ method”) and that is excellent indevice characteristics, and to a method of producing such a high-qualitysilicon single crystal.

BACKGROUND ART

A variety of methods are available to grow silicon single crystals foruse as semiconducting materials. Among these methods, the CZ method isextensively used.

FIG. 1 is a schematic sectional view of a single crystal producingapparatus used for producing single crystals by a normal CZ method. Asshown in FIG. 1, a crucible 1 comprises a quartz-made, bottomed,cylindrical inner layer holding container 1 a, and a graphite-made,similarly bottomed and cylindrical outer layer holding container 1 bthat is fitted over the outside of the inner layer holding container 1a. The constructed crucible 1 is supported by a support shaft 1 c thatis rotated at a predetermined speed. Outside the crucible 1 is set aheater 2, which is provided in the form of a concentric cylinder. Thecrucible 1 is charged with a melt 3 that is a raw molten material heatedby the heater 2. A pulling shaft 4, such as a pull rod or a wire, isprovided at the center of the crucible 1. A seed chuck and a seedcrystal 5 are attached to the distal end of the pulling shaft 4, and theseed crystal 5 is brought into contact with the surface of the melt 3 inorder to grow a single crystal 6. Further, by pulling the seed crystal 5at a predetermined rate using the pulling shaft 4 while rotating thepulling shaft 4 in a direction opposite to the direction of the crucible1 rotated by the support shaft 1 c, the melt 3 is solidified at thedistal end of the seed crystal 5, thereby gradually growing the singlecrystal 6.

For single crystal growth, a seed-constricting step is carried out firstso as to make the crystal dislocation-free. Thereafter, to secure a bodydiameter of the single crystal, a shoulder is formed, and when the bodydiameter has been obtained, a shoulder-changing step is performed. Then,the single crystal growing process is shifted to the single crystalbody-growing step while maintaining the obtained body diameter. When thesingle crystal has been grown to a predetermined length whilemaintaining the body diameter, a tail constricting step is carried outso as to separate the single crystal from the melt in thedislocation-free state. Thereafter, the single crystal separated fromthe melt is taken out of the puller, and cooled under a predeterminedcondition, and processed into wafers. The wafers thus processed from thesingle crystal are used as substrate materials for the preparation ofvarious devices.

In an in-plane area of a wafer that is processed through theabove-described steps, there may occur, in some cases, oxidation-inducedstacking faults (hereinafter referred to as “OSF”) as defects appearingthrough heat treatments. Ring-like extending OSF (hereinafter referredto as “R-OSF”) may appear in some cases depending on the pullingcondition of a single crystal. At the same time, there occur, in thein-plane area of the wafer, defects that called “grown-in defects.”These grown-in defects are formed during single crystal growth anddetected in wafers subjected to heat treatments or predeterminedevaluation processes.

FIG. 2 schematically illustrates a generally observed relationshipbetween the pulling rate during single crystal growth and the positionswhere crystal defects occur. As shown in FIG. 2, in a silicon singlecrystal grown by the CZ method, the region where R-OSF appear shrinksinward from the outer edge of the crystal as the pulling rate isdecreased. Therefore, when a single crystal is grown fast, the crystalin the inner region of R-OSF expands into the whole wafer, while when asingle crystal is grown slowly, the crystal in the outer region of R-OSFexpands into the whole wafer.

Grown-in defects observed on a surface of a wafer are different betweena rapidly grown crystal and a slowly grown crystal. In the crystal thatis grown fast, i.e., in the inner region of R-OSF, defects called “laserscattering tomography defects” (they are also called as “COP” and “FPD,”and are detected by different evaluation methods, but are derived fromthe same kind of defect) are detected. On the other hand, in the crystalthat is grown slowly, i.e., in the outer region of R-OSF, defects called“dislocation clusters” are detected.

FIG. 3 schematically illustrates an example of a typical distribution ofdefects observed at an in-plane position A of the crystal of FIG. 2previously described. This schematically shows the results ofobservations made through X-ray topography as to the distribution ofdefects of a wafer after the wafer was sliced from a single crystalimmediately after growth, had Cu deposited thereon while immersed intoan aqueous solution of copper nitrate, and heat-treated for 20 minutesat 900° C. That is, in the in-plane area of the wafer, R-OSF appears ata position that is about ⅔ of the outside diameter, and laser scatteringtomography defects are found inside R-OSF. Further, an oxygenprecipitation-promoting region exists immediately outside R-OSF so as totouch R-OSF. Oxygen precipitates easily form in this region. Around theouter edge of the wafer extends a region where dislocation clusterseasily occur. Furthermore, it is observed that a denuded zone free ofdislocation clusters is slightly present immediately outside the oxygenprecipitation promoting region, and a denuded zone free of laserscattering tomography defects is slightly present inside R-OSF so as totouch the ring.

OSF impair electrical properties, e.g., in the form of increased leakcurrent while showing themselves up in a high-temperature thermaloxidation process during device fabrication, and dislocation clustersalso greatly deteriorate device characteristics. Therefore, a singlecrystal is usually produced by adjusting the growing rate so that R-OSFis located around the outer edge of a wafer. On the other hand, laserscattering tomography defects are factors for deteriorating the initialoxide film withstand voltage characteristics, and they must also beminimized.

As described earlier, to suppress the occurrence of R-OSF on a surfaceof a wafer, a single crystal is usually grown under such a conditionthat the R-OSF position is limited within the outer edge of the wafer.However, it is known that the R-OSF position is determined, in additionto the pulling rate, by the highest temperature range (from the meltingpoint to 1250° C.) in which the crystal stays during growth, and ishence affected by the heat history of the crystal in the highesttemperature range during pulling. Thus, to determine the R-OSF position,attention must be paid to two factors, i.e., the temperature gradientsin the direction of the pulling shaft and the pulling rate, which are tobe achieved while a single crystal being grown stays in the highesttemperature range. That is, the R-OSF position can be limited around theouter edge of a wafer by decreasing the temperature gradients when thepulling rate is not changed, or by decreasing the pulling rate when thetemperature gradients are not changed.

To check the position and width of R-OSF occurring in the in-plane areaof a wafer, it is effective to observe the distribution of defects inthe wafer through X-ray topography after immersing the wafer that isprocessed from an as-grown single crystal into an aqueous solution ofcopper nitrate to thereby deposit Cu thereon, and heat-treating it for20 minutes at 900° C. Further, the position of the previously describedoxygen precipitation-promoting region present immediately outside R-OSFcan also be checked through a similar method.

When a silicon single crystal has low oxygen content of, e.g., 13×10¹⁷atoms/cm³ or less, one may not observe R-OSF clearly with theabove-described method in some cases. In such cases, it is suggestedthat a ring-like region where the amount of oxygen precipitates is smallbe observed through X-ray topography after a wafer processed from anas-grown single crystal is charged into a heat treatment furnace of 650°C., thereafter heated at a rate of 8° C./min or less, and thenheat-treated for 20 hours at 900° C. and for 10 hours at 1000° C.Further, the position and width of the oxygen precipitation-promotingregion present immediately outside R-OSF can also be checked through asimilar method.

Further, the R-OSF position can also be checked by using the outsidediameter of a circular region where laser scattering tomography defectsare detected as a reference when a wafer processed from an as-grownsingle crystal is subjected to infrared scattering tomography to measurethe laser scattering tomography defects. Furthermore, the density ofdislocation clusters is observed using an optical microscope through theso-called “Secco etching” in which the surface of a specimen wafer isetched using a Secco solution.

Owing to the recent trends not only toward low-temperature processingduring device production to get rid of unsatisfactory effects of OSFeasily occurring in high-temperature processes, but also toward loweroxygen contents in crystals, R-OSF are not considered so serious aproblem as a factor for deteriorating device characteristics. On theother hand, of the grown-in defects, both laser scattering tomographydefects and dislocation clusters are factors for deteriorating devicecharacteristics, and thus it is more important to reduce the density ofthese grown-in defects in the in-plane area of a wafer. While thegrown-in defects are less dense at the previously described denudedzones adjacent to R-OSF, such zones are limited to very narrow regions.

Various methods have so far been proposed to reduce the density ofgrown-in defects in the in-plane area of a wafer. For example, JapaneseUnexamined Patent Application Laid-Open No. 8-330316(1996) proposes amethod in which only the outer region of R-OSF is expanded into thewhole in-plane area of a crystal without causing dislocation clusters tooccur by controlling the pulling rate and the temperature gradientswithin the crystal during single crystal growth. However, according tothe proposed method, both extremely limited in-plane temperaturegradient and pulling conditions must be satisfied at the same time, andthus new improvements must be made in growing silicon single crystalsfor which larger diameter and mass production are called.

Next, Japanese Unexamined Patent Application Laid-Open No.7-257991(1995) and Journal of Crystal Growth 151 (1995, pp. 273-277)disclose methods in which the temperature gradients in the direction ofthe pulling shaft of a single crystal are increased, so that R-OSF candisappear into the inside of the crystal under high-speed pullingconditions, and thus the outer region of R-OSF can be expanded into thewhole in-plane area of the crystal. However, the methods disclosed bythese publications have given no considerations to the distribution oftemperature gradients in the in-plane area of the crystal, i.e., theuniformity of a temperature distribution in the in-plane area of a waferand to the in-plane homogenization of introduced point defects. In otherwords, no considerations are given to means for reducing grown-indefects in the in-plane area of a wafer, and thus, even if only R-OSFare shrunk inward, dislocation clusters do remain in the in-plane areaof the wafer as in conventional crystals. Therefore, the methodsdisclosed in these publications are not successful either in processingwafers having a lower density of grown-in defects.

DISCLOSURE OF THE INVENTION

This invention has been made in view of the above-described conventionalproblems over crystal defects. It is, therefore, an object of theinvention to provide a high-quality silicon single crystal in whichregions free of grown-in defects such as laser scattering tomographydefects and dislocation clusters can be expanded into the in-plane areaof a wafer by controlling the position and width of R-OSF whileadjusting single crystal growing conditions. A further object of theinvention is to provide a high-quality silicon single crystal that canbe grown into a large diameter and a long size. This invention that hasbeen accomplished to achieve the above objects has as its gist thefollowing first to fifth high-quality silicon single crystals andmethods of producing such high-quality silicon single crystals.

1. First High-Quality Silicon Single Crystal

(1) A high-quality silicon single crystal grown by a CZ method,characterized in that the width of R-OSF exceeds 8% of the radius of thegrown crystal and dislocation clusters are absent; or

(2) A high-quality silicon single crystal grown by a CZ method,characterized in that the width of R-OSF exceeds 8% of the radius of thegrown crystal, the inside diameter of the R-OSF is within a range of0-80% of the diameter of the grown crystal, and dislocation clusters arepresent at a low density or absent.

2. Second High-Quality Silicon Single Crystal

(1) A high-quality silicon single crystal grown by a CZ method,characterized in that the outside diameter and the inside diameter of aregion where ring-like extending oxidation-induced staking faults occurare within a range of 0-80% and within a range of 0-33% of the diameterof said grown crystal, respectively, and dislocation clusters areabsent;

(2) A high-quality silicon single crystal grown by a CZ method,characterized in that the inside diameter of a ring-like oxygenprecipitation promoting region is within a range of 0-80% of thediameter of said grown crystal, the inside diameter of a region wherering-like extending oxidation-induced stacking faults occur which is inthe inner side of said oxygen precipitation promoting region is within arange of 0-33% of the diameter of said grown crystal, and dislocationclusters are absent; or

(3) A high-quality silicon single crystal grown by a CZ method,characterized in that the outside diameter and the inside diameter of aring-like region where the amount of oxygen precipitates is small arewithin a range of 0-80% and within a range of 0-33% of the diameter ofsaid grown crystal, respectively, and dislocation clusters are absent.

3. Third High-Quality Silicon Single Crystal and Method of Producing theSame

(1) A high-quality silicon single crystal grown under such a conditionthat the crystal stays in a temperature range of 1250° C.-1000° C. for 7hours or more when pulled by a CZ method, characterized in that theoutside diameter of R-OSF is within a range of 0-60% of the diameter ofthe grown crystal, and a method of producing this single crystal.

(2) A high-quality silicon single crystal grown under such a conditionthat the crystal stays in a temperature range of 1250° C.-1000° C. for 7hours or more when pulled by a CZ method, characterized in that theinside diameter or the outside diameter of an oxygen precipitationpromoting region is within a range of 0-60% of the diameter of the growncrystal, and a method of producing this single crystal.

(3) A high-quality silicon single crystal grown under such a conditionthat the crystal stays in a temperature range of 1250° C.-1000° C. for 7hours or more when pulled by a CZ method, characterized in that theoutside diameter of a ring-like region where the amount of oxygenprecipitates is small is within a range of 0-60% of the diameter of thegrown crystal, and a method of producing this single crystal.

(4) A high-quality silicon single crystal grown under such a conditionthat the crystal stays in a temperature range of 1250° C.-1000° C. for 7hours or more when pulled by a CZ method, characterized in that theoutside diameter of a circular region where laser scattering tomographydefects are detected is within a range of 0-60% of the diameter of thegrown crystal, and a method of producing this single crystal.

4. Fourth High-Quality Silicon Single Crystal and Method of Producingthe Same

(1) A high-quality silicon single crystal grown under such a conditionthat a temperature gradient in the vertical direction parallel with apulling shaft of the crystal is smaller at the outer edge than at thecenter and is 2.6° C./mm or more at the center when the crystal stays ina temperature range of its solidifying point to 1250° C. while pulled bya CZ method, characterized in that the outside diameter of R-OSF iswithin a range of 0-60% of the diameter of the grown crystal, and amethod of producing this single crystal; and

(2) A method of producing a high-quality silicon single crystalcharacterized in that the single crystal is grown under such conditionsthat a temperature gradient in the vertical direction parallel with apulling shaft of the crystal is smaller at the outer edge than at thecenter and is 2.6° C./mm or more at the center when the crystal stays atemperature range of its solidifying point to 1250° C. during growth,and that the outside diameter of R-OSF is within a range of 0-60% of thediameter of the grown crystal.

5. Fifth High-Quality Silicon Single Crystal and Method of Producing theSame

(1) A high-quality silicon single crystal grown in such a state that theshape of a solid-melt interface between the single crystal and a melt isflat or upwardly convex when pulled by a CZ method, characterized inthat the outside diameter of R-OSF is within a range of 0-60% of thediameter of the grown crystal; and

(2) A method of producing a high-quality silicon single crystalcharacterized in that the single crystal is pulled in such a state thatthe shape of a solid-melt interface between the single crystal beinggrown and a melt is flat or upwardly convex at such a low rate as toallow the outside diameter of R-OSF occurring in the single crystal tobe within a range of 0-60% of the diameter of the crystal. In thisproducing method, it is desirable to set the rotating speed of acrucible at 5 rpm or less, or/and the rotating speed of the singlecrystal at 13 rpm or more.

In this invention, the distribution of each type of defect may bedetected through X-ray topography after immersing an as-grown wafer orspecimen into an aqueous solution of copper nitrate to thereby depositCu thereon and then heat-treating it for 20 minutes at 900° C. Further,when the oxygen concentration is decreased, the distribution of OSF maynot be observed satisfactorily in some cases under this condition. Insuch cases, X-ray topography may be used after charging an as-grownwafer or specimen into a furnace whose temperature has reached about650° C., heating it up to 900° C. at a rate of 5° C./min, soaking it for20 hours, thereafter heating it to 1000° C. at a rate of 10° C./min, andsoaking it for 10 hours at that temperature. The density of dislocationclusters is detected by subjecting the surface of the wafer or specimento Secco etching and observing its defects using an optical microscope.Further, laser scattering tomography defects are detected through laserscattering tomograpy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a single crystal producingapparatus that is used for producing a single crystal by a normal CZmethod.

FIG. 2 schematically illustrates a generally observed relationshipbetween the pulling rate during single crystal growth and the positionswhere crystal defects occur, and FIG. 3 schematically illustrates anexample of a typical distribution of defects observed at an in-planeposition A of the crystal of FIG. 2.

FIG. 4 schematically shows the relationship between the radial positionand width of R-OSF and how dislocation clusters occur in an 8″-diametercrystal grown under conventional growing conditions, and FIG. 5schematically shows the relationship between the radial position andwidth of R-OSF and how dislocation clusters occur in an 8″-diametercrystal grown under improved growing conditions.

FIG. 6 schematically shows the relationship between the distribution ofconcentrations of vacancies introduced into an in-plane area of acrystal and the width of R-OSF formed in the crystal.

FIG. 7 shows the relationship between the in-plane R-OSF position andthe FPD distribution density in an as-grown crystal that was grown inaccordance with Example 1, and FIG. 8 shows the results of examinationsmade as to time-zero dielectric breakdown (TZDB) of wafers processedfrom the single crystal that was produced in accordance with Example 1.

FIG. 9 shows the FPD distribution density in an as-grown crystal thatwas grown in accordance with Example 2.

FIG. 10 shows the relationship between the in-plane R-OSF position andthe FPD distribution density in an as-grown crystal that was grown inaccordance with Example 3, and FIG. 11 shows the results of examinationsmade as to time-zero dielectric breakdown (TZDB) of wafers processedfrom the single crystal that was produced in accordance with Example 3.

FIG. 12 shows the FPD distribution density in an as-grown crystal thatwas grown in accordance with Example 4.

FIG. 13 shows a pulling rate changing pattern in the case where thepulling rate was changed in the course of growing single crystals, andFIG. 14 shows the heat histories of crystals A and B whose pulling rateswere changed in the course of their growth.

FIG. 15 shows the results of examinations made as to time-zerodielectric breakdown (TZDB) of wafers processed from a single crystalthat was produced in accordance with Example 6.

FIGS. 16(a) and 16(b) illustrate the temperature distributions of singlecrystals being grown and the behaviors of vacancies resulting from suchtemperature distributions.

FIGS. 17(a), 17(b), and 17(c) show how the concentration distribution ofvacancies and that of interstitial Si atoms in the in-plane area of asingle crystal change near the solid-melt interface and in a temperaturerange of the melting point to 1250° C. during single crystal growth.

FIGS. 18(a), 18(b) and 19 schematically show the results of observationsmade through X-ray topography as to single crystals that were pulled inaccordance with Examples 9 and 10 after the single crystals were splitlengthwise in the as-grown state, coated with Cu, and heat-treated at900° C. to thereby render their defective regions visible by type,respectively.

FIG. 20 shows the results of observations made through Secco etching asto the FPD distribution density in the as-grown crystal that was grownin accordance with Example 10, and FIG. 21 shows the results ofmeasurements made through infrared scattering tomography as to thedistribution density of laser scattering tomography defects in theas-grown crystal that was grown in accordance with Example 10.

FIG. 22 schematically shows the results of observations made throughX-ray topography as to a single crystal that was pulled in Example 11after the single crystal was sliced in the as-grown state, coated withCu, and heat-treated at 900° C. to thereby render its defective regionsvisible by type, and FIG. 23 shows the results of examinations made asto time-zero dielectric breakdown (TZDB) of wafers processed from thesingle crystal that was pulled in Example 11.

FIG. 24 schematically shows the results of observations made throughX-ray topography as to a single crystal that was pulled in Example 12after the single crystal was sliced in the as-grown state, coated withCu, and heat-treated at 900° C. to thereby render its defective regionsvisible by type.

FIGS. 25(a) and 25(b) schematically illustrate the temperaturedistributions in in-plane areas of single crystals that are beingpulled.

FIG. 26 schematically shows a cross section of a single crystalproducing apparatus used in Example 13, and FIG. 27 shows the results ofexaminations made as to the defect distribution in accordance withExample 13.

FIGS. 28(a) and 28(b) schematically show the solid-melt interfaces ofsingle crystals that are being pulled.

FIGS. 29(a) and 29(b) illustrate the relationship between thetemperature gradients at the center and at the surface in a singlecrystal that is being pulled and the densities of vacancies andinterstitial atoms.

FIGS. 30(a), 30(b), and 30(c) show the results of examinations made asto the defect distribution in the case where the rotating speed of acrucible is varied at levels of 10 rpm, 3 rpm and 1 rpm.

FIG. 31 shows the results of examinations made as to Example 15.

BEST MODE FOR CARRYING OUT THE INVENTION

First to fifth high-quality silicon single crystals and methods ofproducing the same according to this invention will be describedspecifically on an itemized basis. Further, in the followingdescription, the position where R-OSF occurs in the in-plane area of awafer crystal is indicated as follows. When r=0, R-OSF occurs at thecenter of the crystal, and when r=R, R-OSF occurs at the outer edge ofthe crystal, where R is the distance (radius) from the center to theouter edge of the crystal, and r is the position where R-OSF occurs inthe radial direction of the crystal. Note that the position of R-OSF isindicated in terms of its inside diameter.

1. First High-Quality Silicon Single Crystal

Concerning a first high-quality silicon single crystal, the inventorsturned their attention to expanding denuded zones present next to R-OSFand near the outer region of R-OSF into a larger area of a wafer byimproving the CZ method-based growing conditions of a single crystal,although R-OSF do occur in the in-plane area of the wafer. To implementthis, the inventors examined how dislocation clusters occur in relationto the position and width of R-OSF occurring in a crystal grown underconventional conditions.

FIG. 4 schematically shows the relationship between the radial positionand width of R-OSF and how dislocation clusters occur in an 8″-diametercrystal grown under the conventional growing conditions (the axis ofabscissa indicates the width of R-OSF in %). From the results of anumber of examinations, it is apparent that when a crystal is grownunder the conventional growing conditions, the R-OSF width is 8% or lessof its radius. FIG. 4 shows that dislocation clusters are observed inthe outer region of R-OSF when the R-OSF width is 8% of the radius ofthe grown crystal and R-OSF are located at r=⅔R. Therefore, when R-OSFoccurs at positions closer toward the center than r=⅔R, dislocationclusters are observed in the outer region of R-OSF. Further, it isunderstood that more dislocation clusters tend to occur as the R-OSFwidth is decreased.

When the R-OSF width in the grown crystal is 8% or less of the radius,shrinking R-OSF toward the center can decrease the density of laserscattering tomography defects in the inner region of R-OSF. Thiscontributes to improving time-zero dielectric breakdown (TZDB). However,dislocation clusters occurring in the outer region of R-OSF deterioratecharacteristics, thus making the crystal unsuitable as a material fordevice substrates.

Next, examinations were made as to the R-OSF positions and widths andhow dislocation clusters occur in a 6″-diameter crystal and an8″-diameter crystal that were grown under the following improved growingconditions in examples to be described later, respectively.

FIG. 5 schematically shows the relationship between the radial positionand width of R-OSF and how dislocation clusters occur in the 8″-diametercrystal that was grown under the improved growing conditions (the axisof abscissa indicates the width of R-OSF in %). It is understood thatthe improved growing conditions can make the R-OSF width wider and theregion free of dislocation clusters expands. For example, it isunderstood from FIG. 5 that when the R-OSF width becomes 30% of thediameter of the crystal, dislocation clusters do not occur no matterwhere R-OSF is located.

FIG. 7, which will be described later, shows the relationship betweenthe in-plane R-OSF position and the FPD distribution density in anas-grown crystal that was grown in accordance with Example 1. That is,FIG. 7 shows the FPD density in an in-plane area of a wafer free ofdislocation clusters in the case where the width of R-OSF was within arange of more than 8% to about 39% of the radius of the grown crystal(the width of R-OSF was 30 mm in the 6″-diameter crystal) and the R-OSFposition was changed. As shown in FIG. 7, as the R-OSF width grows, FPDare sometimes observed around the center when the R-OSF position isr=⅔R. However, when the R-OSF position is changed to r=⅓R, there are noobservable FPD.

FIG. 10, which will be described later, shows the relationship betweenthe in-plane R-OSF position and the FPD distribution density in anas-grown crystal that was grown in accordance with Example 3. Similarlyto FIG. 7 described above, FIG. 10 shows the FPD density in an in-planearea of a wafer free of dislocation clusters in the case where the R-OSFwidth was about 39% of the radius of the grown crystal (the R-OSF widthwas 40 mm in the 8″-diameter crystal) and the R-OSF position waschanged. As is apparent from FIG. 10 that as the R-OSF width grows, noFPD are observed in the inner region of R-OSF depending on the R-OSFposition. Thus, increased R-OSF widths can eliminate dislocation clusterformation, and reduce the laser scattering tomography defect density inthe inner region of R-OSF to an extremely small level, and even caneliminate the formation of laser scattering tomography defects by addingsome other conditions.

According to the investigations made by the inventors, to eliminategrown-in defects from the in-plane area of a crystal without causingdislocation clusters and laser scattering tomography defects to occur inthe in-plane area of a wafer, it is required that the R-OSF width beincreased and that the R-OSF position be within a range of 0-80% of thediameter of the crystal.

That is, compared with the conventional crystal, R-OSF is located closerto the center of the in-plane area of the wafer, but it is wider so thatdenuded zones are expanded with no dislocation clusters present in theouter region of R-OSF, and in addition, laser scattering tomographydefects are no longer observed in the inner region of R-OSF. Sincegrown-in defects that deteriorate device characteristics can beprevented from occurring in the whole in-plane area of a wafer in thisway, the percent nondefective of devices can be greatly improved.

The reason why the R-OSF width is set at values exceeding 8% of theradius of the grown crystal in the first high-quality silicon singlecrystal is as follows. In the conventional example, dislocation clustersoccur with the R-OSF position located at r=⅔R when the R-OSF width is 8%or less, while when this invention is applied, dislocation clusters nolonger appear even if the R-OSF width exceeds 8% and the R-OSF region isshrunk to be positioned at r=⅔R or less. Further, the reason why theR-OSF position is set within a range of 0-80% of the diameter of thegrown crystal is because within such a range, grown-in defects can bereduced to an extremely small degree or totally eliminated. For example,the smaller the value r, the less dense grown-in defects, with nogrown-in defects detected when r=⅓R or less. Therefore, in the firstsilicon single crystal of this invention, it is specified that grown-indefects are present “at a low density or absent,” because its grown-indefects are reduced markedly compared with the conventional example inwhich the whole crystal is found inside R-OSF.

To produce the first high-quality silicon single crystal, it is requiredat the time of CZ method-based growth that the heat history of a singlecrystal during a period in which the crystal stays in a high-temperaturerange (from the melting point to 1200° C.) be controlled so as to makethe concentrations of point defects that are introduced into an in-planearea of the crystal during that period uniform in such in-plane area.These point defects in the crystal are classified into vacancies andinterstitial Si atoms. Vacancies are particularly relevant to R-OSFformation, and the position and width of R-OSF occurring in the in-planearea of a crystal coincide with the site and region where the vacancyconcentration is within a certain limited range.

During single crystal growth, the in-plane temperature gradients of acrystal usually differ in the direction of the pulling shaft.Specifically, the crystal temperature drops faster toward the outeredge, resulting in larger temperature gradients toward the outer edge.In this case, vacancies introduced into the crystal diffuse toward thesolid-melt interface in the direction of the pulling shaft and disappearin larger amounts with increasing temperature gradient, resulting in amarkedly reduced concentration of the vacancies that remain asintroduced into the crystal. As a result, when the temperature gradientsin the direction of the pulling shaft differ, the concentrations ofvacancies introduced into the in-plane area of the crystal are notuniform, resulting in lower vacancy densities toward the outer edge ofthe crystal. Therefore, by making the temperature gradients in thedirection of the pulling shaft in the in-plane area of the crystaluniform, the in-plane vacancy concentrations can be made uniform.

FIG. 6 schematically shows the relationship between the distribution ofconcentrations of vacancies introduced into an in-plane area of acrystal and the width of R-OSF formed in the crystal. The axis ofordinate in FIG. 6 indicates the vacancy concentration C_(V) and theaxis of abscissa the in-plane position of the crystal. Further, the leftside of FIG. 6 indicates cases where vacancy concentrations fluctuategreatly from the in-plane center toward the outer edge, and the rightside cases where vacancy concentrations are comparatively uniform. SinceR-OSF occurs at a region where the vacancy concentration is within alimited range, the R-OSF width widens when a relatively uniform in-planevacancy concentration is achieved as indicated by the right side of FIG.6. As described earlier, the in-plane R-OSF width in the conventionalcrystal is kept at 8% or less of the radius of the grown crystal. Thisis because the vacancy concentration range coinciding with the R-OSFoccurring region is limited within 8% of the radius of the grown crystalsince the temperature gradients in the direction of the pulling shaft inthe in-plane area of the crystal are not uniform under the conventionalgrowing conditions.

By improving the growing conditions, e.g., by improving heating means,heat insulating members and the like for the hot zone of a singlecrystal producing apparatus, the temperature gradients in the directionof the pulling shaft in an in-plane crystal area are made uniform and,by doing so, the amounts of vacancies introduced into the in-plane areaare made uniform. As a result, the vacancy concentration range withinwhich R-OSF is formed can be increased, and hence the R-OSF width can bewidened correspondingly. Further, the denuded zone formed closely in theouter region of R-OSF can also be expanded by making the amounts ofvacancies introduced into the in-plane area uniform so that the vacancyconcentration range within which the denuded zone is formed isincreased. By doing so, regions free of grown-in defects such as laserscattering tomography defects and dislocation clusters that deterioratedevice characteristics are expanded into the whole in-plane area of thecrystal, whereby high-quality wafers excellent in device characteristicscan be obtained.

First high-quality silicon single crystals of this invention wereproduced in two different diameters, 6″ and 8″, and the forms of R-OSFappearing in these crystals and the quality characteristics of thesecrystals were examined as Examples 1 to 4.

1-1. EXAMPLE 1

A 6″-diameter single crystal was produced using the single crystalproducing apparatus shown in FIG. 1. The crucible was charged with 60 kgof polysilicon that is a raw material for the preparation of a crystal,and boron was further added as a p-type dopant so as to obtain anelectrical resistivity of 10 Ωcm.

After achieving a 10-torr Ar atmosphere within the chamber, the power ofthe heater is adjusted so as to melt all the raw material for thecrystal. After stabilizing the melt within the crucible, the lower endof a seed crystal was immersed into the melt, and the single crystal waspulled while rotating the crucible and the pulling shaft.

In Example 1, it is an object to examine how the widths of R-OSF and adenuded zone change, or how the FPD density changes in accordance withthe growing conditions. To do so, growing rate changing tests in whichthe growing rate of the crystal was gradually decreased were conductedby growing the crystal in a hot zone where the conventional temperaturedistribution in an in-plane area of a crystal was improved so that theamounts of vacancies introduced into the in-plane area of the crystalbecome uniform.

To carry out these tests, the power of the heater was adjusted when thesingle crystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatR-OSF occur around the outer edge by increasing the pulling rate at theinitial stage. When the single crystal was pulled to a length of 100 mm,the pulling rate was gradually slowed down, and the forms of R-OSF and adenuded zone, as well as the behavior of FPD formed in the inner regionof R-OSF were examined.

The form of R-OSF was examined by splitting the as-grown crystal thatwas grown in accordance with Example 1 lengthwise, coating the splitcrystal with Cu, heat-treating it at 900° C. to thereby render visibleits defective regions by type, and thereafter taking X-ray topographicpictures. The widths of R-OSF and the denuded zone were much wider thanthose of the conventional crystal. While the expected width of R-OSFcircularly formed in the in-plane area was 30 mm on one side, the R-OSFwidth was expanded to a maximum of 40 mm, with a total width of 80 mm onboth sides found at a site. That is, there was even a site where morethan half (52%) the radius of the 6″-diameter crystal corresponded tothe R-OSF width. It was also verified that even if R-OSF were caused tooccur around the center of the crystal to thereby expand their outerregion, no dislocation clusters were observed because the denuded zoneexpanded greatly.

FIG. 7 shows the relationship between the in-plane R-OSF position andthe FPD distribution density in the as-grown crystal that was grown inaccordance with Example 1. Note that Secco etching was effected toobserve the in-plane R-OSF position. Further, the R-OSF width wascontrolled to 30 mm, which was on the order of 39% of the radius of thecrystal. As is apparent from FIG. 7 that FPD were observed around thecenter of the crystal when the in-plane R-OSF position was r=⅔R, whileno FPD were observed when the R-OSF position was changed to r=⅓R.Therefore, by controlling the width and in-plane position of R-OSF whileadjusting the growing conditions, one can grow crystals in the in-planearea of which grown-in defects such as laser scattering tomographydefects (FPD) and dislocation clusters are not observed.

FIG. 8 shows the results of examinations made as to time-zero dielectricbreakdown (TZDB) of wafers processed from the single crystal that wasproduced in accordance with Example 1. That is, FIG. 8 shows an averagepercent nondefective at every R-OSF position that was varied from thecenter to the outer edge with the R-OSF width being 30 mm. It can besaid from FIG. 8 that the percent nondefective in terms of TZDB in thein-plane crystal area was 95% or more in the case where the thickness ofan oxide film was 25 nm, the applied voltage was 8 M/V, the R-OSFposition was r=⅓R, and the FPD density was very small.

1-2. EXAMPLE 2

In Example 2, a 6″-diameter crystal was grown at such a pulling ratethat the in-plane R-OSF position is r=⅓R, and how R-OSF, a denuded zone,and the FPD density change in accordance with the growing conditions wasexamined. To do so, the crystal was grown in the same hot zone as inExample 1 so that the amounts of vacancies introduced into an in-planearea of the crystal become uniform.

After stabilizing the melt within the crucible under the same conditionsas in Example 1, the power of the heater was adjusted when the singlecrystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatthe pulling rate is high at the initial stage and thus R-OSF occursaround the outer edge. When pulled to a length of 100 mm, the singlecrystal was then grown at such a pulling rate that the in-plane R-OSFposition is r=⅓R, and the behaviors of R-OSF, the denuded zone, and theFPD formed in the inner region of R-OSF were examined at different sitesof the crystal.

The form of R-OSF and how grown-in defects occur were examined byapplying Cu onto a wafer processed from the as-grown crystal that wasgrown in accordance with Example 2, heat-treating it at 900° C. tothereby render visible its defective regions by type, and thereaftertaking X-ray topographic pictures. It was verified that the generatedR-OSF had a width of 30 mm and its inside diameter was r=⅓R. It isunderstood from this that the widths of R-OSF and the denuded zone weregreatly expanded compared with the conventional crystal.

FIG. 9 shows the FPD distribution density in the as-grown crystal thatwas grown in accordance with Example 2. Note that Secco etching waseffected to observe the in-plane R-OSF position. Further, the R-OSFwidth was controlled to 30 mm, which was on the order of 39% of theradius of the crystal. When the in-plane R-OSF position was r=⅓R, no FPDwere observed at all, nor were dislocation clusters observed, either.

1-3. EXAMPLE 3

An 8″-diameter single crystal was produced using the single crystalproducing apparatus shown in FIG. 1. The crucible was charged with 120kg of polysilicon that is a raw material for the preparation of acrystal, and boron was further added as a p-type dopant so as to obtainan electrical resistivity of 10 Ωcm. After achieving a 10-torr Aratmosphere within the chamber, the power of the heater was adjusted soas to melt all the raw material for the crystal. After stabilizing themelt within the crucible, the lower end of a seed crystal was immersedinto the melt, and the single crystal was pulled while rotating thecrucible and the pulling shaft.

First of all, growing condition changing tests in which the pulling ratewas gradually decreased were conducted in order to examine how thewidths of R-OSF and a denuded zone change, or how the FPD densitychanges in accordance with the growing conditions similarly to Example1.

To carry out these tests, the power of the heater was adjusted when thesingle crystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatthe pulling rate is high at the initial stage and thus R-OSF occursaround the outer edge. When the single crystal was pulled to a length of100 mm, the pulling rate was gradually decreased, and the forms of R-OSFand the denuded zone, as well as the behavior of FPD formed in the innerregion of R-OSF were examined.

The forms of R-OSF and the denuded zone were examined by splitting theas-grown crystal that was grown in accordance with Example 3 lengthwise,coating the split crystal with Cu, heat-treating it at 900° C. tothereby render visible its defective regions by type, and thereaftertaking X-ray topographic pictures. The results were similar to those ofExample 1 described previously, and the widths of R-OSF and the denudedzone were greatly expanded compared with the a conventional crystal. TheR-OSF width was as wide as 40 mm on one side, totaling 80 mm on bothsides. That is, nearly half (39%) of the radius of the 8″-diametercrystal corresponded to the R-OSF region.

FIG. 10 shows the relationship between the in-plane R-OSF position andthe FPD distribution density in the as-grown crystal that was grown inaccordance with Example 3. Note that Secco etching was effected toobserve the in-plane R-OSF position. Further, the R-OSF width wascontrolled to 40 mm, which was on the order of 39% of the radius of thecrystal. As is understood from FIG. 10 that FPD were observed around thecenter of the crystal when the R-OSF position was r=⅖R, while no FPDwere observed when r=⅓R or less. Therefore, by controlling the width andin-plane position of R-OSF while adjusting the growing conditions, onecan grow a crystal in the in-plane area of which laser scatteringtomography defects (FPD) are present in a markedly smaller density thanbefore or not observed, and grown-in defects such as dislocationclusters are not observed, either.

FIG. 11 shows the results of examinations made as to time-zerodielectric breakdown (TZDB) of wafers processed from the single crystalthat was produced in accordance with Example 3. That is, FIG. 11 showsan average percent nondefective at every R-OSF position that was variedfrom the center to the outer edge with the R-OSF width being 40 mm. Itcan be said from FIG. 11 that the percent nondefective in terms of TZDBin the in-plane crystal area was 95% or more in the case where thethickness of an oxide film was 25 nm, the applied voltage was 8 M/V, theR-OSF position was r=⅓R, and the FPD density was very small.

1-4. EXAMPLE 4

In Example 4, an 8″-diameter crystal was grown at such a pulling ratethat the in-plane R-OSF position is r=⅓R, and how R-OSF, a denuded zone,and the FPD density change in accordance with the growing conditions wasexamined. To do so, the crystal was grown in the same hot zone as inExample 3 so that the amounts of vacancies introduced into an in-planearea of the crystal become uniform.

After stabilizing the melt within the crucible under the same conditionsas in Example 3, the power of the heater was adjusted when the singlecrystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatthe pulling rate is high at the initial stage and thus R-OSF occursaround the outer edge. When pulled to a length of 100 mm, the singlecrystal was then pulled at such a rate that the in-plane R-OSF positionis r=⅓R until the crystal grew to a length of 1000 mm. Thereafter, thebehaviors of R-OSF, the denuded zone, and the FPD formed in the innerregion of R-OSF were examined at different sites of the crystal.

It was verified that the R-OSF width was 40 mm and its inside diameterwas r=⅓R after coating with Cu a wafer processed from the as-grownsingle crystal that was grown in accordance with Example 4,heat-treating it at 900° C. to thereby render visible its defectiveregions by type, and thereafter taking X-ray topographic pictures.Similarly to Example 3 described previously, it is understood that theR-OSF width and the denuded zone were greatly expanded compared with theconventional crystal, and that no dislocation clusters occurred.

FIG. 12 shows the FPD distribution density in the as-grown crystal thatwas grown in accordance with Example 4. Note that Secco etching waseffected to observe the in-plane R-OSF position. Further, the R-OSFwidth was controlled to 40 mm, which was on the order of 39% of theradius of the crystal. When the in-plane R-OSF position was r=⅓R,neither FPD nor dislocation clusters were observed at all.

As described in the foregoing, according to the first high-qualitysilicon single crystal of this invention, the width of R-OSF occurringin its in-plane area can be expanded, and regions free of grown-indefects, such as laser scattering tomography defects and dislocationclusters, can also be expanded. Therefore, a semiconducting materialexcellent in device characteristics can be supplied. In addition, sincethe single crystal according to this invention is grown by achieving auniform concentration of point defects introduced into the in-plane areaof the crystal, single crystals of a large diameter and a long size canbe produced, and thus the production cost can be reduced and the growingefficiency improved.

2. Second High-Quality Silicon Single Crystal

In inventing a second high-quality silicon single crystal, the inventorsexamined the behavior of R-OSF that appear in the in-plane area of asingle crystal in the case where the heat history of each crystal siteis changed by changing the pulling rate in the course of growing thesingle crystal.

FIG. 13 shows a pulling rate-changing pattern in the case where thepulling rate was changed in the course of growing single crystals. Asshown in FIG. 13, a crystal A was grown to a length of 500 mm under sucha condition that the initial pulling rate is 0.7 mm/min and R-OSF occurat r=½R. The crystal A was then grown to a length of 550 mm byincreasing the pulling rate to 1.2 mm/min, and thereafter the pullingrate was dropped to 0.7 mm/min, at which pulling rate the crystal A wasgrown until it was subjected to a tail constricting step at a length of850 mm.

On the other hand, a crystal B was grown to a length of 500 mm undersuch a condition that the initial pulling rate is 0.7 mm/min and R-OSFoccurs at r=½R. The crystal B was then grown to a length of 550 mm bydropping the pulling rate to 0.2 mm/min, and thereafter the pulling ratewas increased to 0.7 mm/min, at which pulling rate the crystal B wasgrown until it was subjected to a tail constricting step at a length of850 mm.

FIG. 14 shows the heat histories of the crystals A and B in which thepulling rates were changed in the course of their growth. As is apparentfrom FIG. 14, a site of the crystal A that is located at a positionequal to or smaller than a crystal length of 500 mm (the positioncorresponding to a single crystal length of 350 mm in FIG. 14) israpidly cooled compared with a crystal that was grown at a constantpulling rate of 0.7 mm/min while such crystal site stays in apredetermined temperature range (980-900° C. in FIG. 14). In contrast, asite of the crystal B that is located at a position equal to or smallerthan 500 mm is slowly cooled compared with the crystal that was grown ata constant pulling rate of 0.7 mm/min while such crystal site stays inthe predetermined temperature range.

In the above-described examinations of the heat histories, examinationswere further made as to the behaviors of R-OSF corresponding to thecases where the crystal sites were cooled at a constant rate, rapidly,and slowly by selecting a high temperature range (1100° C.-1000° C.) andan intermediate temperature range (980° C.-900° C.) as the predeterminedtemperature ranges of the crystals A and B. As a result, R-OSF did occurat certain widths in the in-plane sites of both crystals that werecooled at the constant rate, respectively. In contrast, the site of thecrystal A that was rapidly cooled while staying in the high temperaturerange (1100° C.-1000° C.) had R-OSF formed whose width was wider thanthe R-OSF formed in the sites that were cooled at the constant rate, butthe site of the crystal A that was rapidly cooled while staying in theintermediate temperature range (980° C.-900° C.) exhibited no R-OSF.

On the other hand, the site of the crystal B that was slowly cooledwhile staying in the high temperature range (1100° C.-1000° C.)exhibited no R-OSF, but its site that was slowly cooled while staying inthe intermediate temperature range (980° C.-900° C.) had R-OSF formedwhose width was wider than the R-OSF formed in the sites that werecooled at the constant rate. Therefore, the R-OSF width fluctuated withdifferent heat histories of the crystals in the high and intermediatetemperature ranges, and the R-OSF behavior in the crystal A wascompletely opposite to that in the crystal B in terms of width.

While no clear theory has been established with respect to theabove-described behavior, such behavior indicates that the width acrosswhich R-OSF occurs in the in-plane area of a wafer depends on the heathistories of a single crystal in the high and intermediate temperatureranges. Further, as single crystals tend to contain less oxygen, R-OSFmay not appear conspicuously in some cases, as has been describedearlier.

Moreover, to produce a high-quality silicon single crystal excellent indevice characteristics, it is important that a uniform concentration ofpoint defects introduced into an in-plane area of a crystal during CZmethod-based growth is achieved in such in-plane area. Thus, aspreviously described, to produce the first high-quality silicon singlecrystal, temperature gradients in the direction of the pulling shaft aremade uniform by improving the growing conditions, and the amounts ofvacancies introduced into the in-plane area of the crystal are hencemade uniform, whereby the R-OSF width is widened. However, differencesin heat history during growth cause the R-OSF width to fluctuate. Forexample, in the case where the crystal site that stays in the hightemperature range (1100° C.-1000° C.) during growth is slowly cooled,the R-OSF width becomes very narrow. On the other hand, in the casewhere the crystal site that stays in the intermediate temperature range(980° C.-900° C.) during growth is rapidly cooled, the R-OSF widthsometimes becomes very narrow, too.

If this is true, it may not be enough, in some cases, to control thegrowing conditions only in terms of the width across which R-OSF occurin the in-plane area of a crystal as in the first high-quality siliconsingle crystal. To overcome this situation, it is necessary to proposethe second high-quality silicon single crystal so that a high-qualitywafer having satisfactory device characteristics can be produced byexpanding regions free of grown-in defects such as laser scatteringtomography defects and dislocation clusters which deteriorate devicecharacteristics into the whole in-plane area of the crystalindependently of the width of R-OSF occurring in the in-plane crystalarea of the wafer.

The second high-quality silicon single crystal of this invention hasbeen invented on the basis of the above-described viewpoint. It is asilicon single crystal grown by CZ method, and includes a silicon singlecrystal in which any of “the outside diameter of a region where R-OSFoccurs,” “the inside diameter of an oxygen precipitation-promotingregion,” and “the outside diameter of a ring-like region where theamount of oxygen precipitates is small” is in a range of 0-80% of thediameter of the grown crystal.

In the second high-quality silicon single crystal, the reason why “theoutside diameter of a region where R-OSF occurs” or “the inside diameterof an oxygen precipitation-promoting region” is used as a reference ofthe R-OSF position is as follows. Since the width across which R-OSFoccurs in the in-plane area of a wafer depends on the heat histories ofa single crystal when the crystal stays in the high and intermediatetemperature ranges, the R-OSF position is controlled by getting rid ofthese factors. Further, the reason why “the outside diameter of aring-like region where the amount of oxygen precipitates is small” isused as a reference is because considerations are given to cases whereR-OSF may not appear conspicuously as single crystals tend to containless oxygen.

Furthermore, the reason why the R-OSF position is limited within therange of 0-80% of the diameter of the grown crystal is because withinsuch a range, grown-in defects can be reduced to an extremely smalllevel or eliminated.

To evaluate the quality of the second high-quality silicon singlecrystal, single crystals were produced in two different diameters, 6″and 8″, and the forms of R-OSF appearing on these crystals and thequality characteristics of these crystals were examined as Examples 5 to8. The results will be described below in comparison with the firsthigh-quality silicon single crystal wafers.

2-1. EXAMPLE 5

A 6″-diameter single crystal was produced using the single crystalproducing apparatus shown in FIG. 1. The crucible was charged with 60 kgof polysilicon that is a raw material for the preparation of a crystal,and boron was further added as a p-type dopant so as to obtain anelectrical resistivity of 10 Ωcm. After achieving an Ar atmospherewithin the chamber, the power of the heater was adjusted so as to meltall the raw material for the crystal. After stabilizing the melt withinthe crucible, the lower end of a seed crystal was immersed into themelt, and the single crystal was pulled while rotating the crucible andthe pulling shaft.

In Example 5, it is an object to examine how the widths of R-OSF and adenuded zone change, or how the FPD density changes under such acondition that a site of a single crystal is slowly cooled while stayingin the temperature range of 1100° C.-1000° C., compared with theconventional growing conditions. To do so, pulling rate changing testswere conducted by improving the conventional temperature distribution inan in-plane crystal area so that the amounts of vacancies introducedinto the in-plane crystal area become uniform, and by growing the singlecrystal in a hot zone where the crystal can be slowly cooled while itstays in a predetermined temperature range.

Examinations were made as to the form of R-OSF by splitting the as-growncrystal that was grown in accordance with Example 5 lengthwise, coatingthe split crystal with Cu, heat-treating it at 900° C. to thereby rendervisible its defective regions by type, and thereafter taking X-raytopographic pictures. Compared with the conventional crystal, the R-OSFwidth and the denuded zone were greatly expanded. The R-OSF widthfluctuated from a maximum of 40 mm to about 6 mm depending on the singlecrystal length. Even if R-OSF did occur in the in-plane crystal area,the outer region of R-OSF similarly expanded, and thus no dislocationclusters occurred. Further, even when R-OSF disappeared into thein-plane crystal area, no dislocation clusters occurred. That is, thesilicon single crystal wafer of this invention can suppress theformation of grown-in defects by controlling where the outside diameteror the inside diameter of R-OSF is located without depending on theR-OSF width.

The FPD distribution density in the as-grown crystal that was grown inaccordance with Example 5 was similar to that shown in FIG. 7 describedpreviously. That is, FPD were observed around the center of the crystalwhen the in-plane R-OSF position was r=⅔R, while no FPD were observedwhen the R-OSF position was changed to r=⅓R.

The results of examinations made as to time-zero dielectric breakdownTZDB) of wafers processed from the single crystal that was produced inaccordance with Example 5 was similar to those of FIG. 8 describedpreviously.

Specifically, an average percent nondefective in terms of in-plane TZDBat the position where R-OSF were present was 95% or more in the casewhere the thickness of an oxide film was 25 nm, the applied voltage was8 M/V, the R-OSF position was r=⅓R, and the FPD density was very small.

2-2. EXAMPLE 6

In Example 6, examinations were made as to how the widths of R-OSF, theoxygen precipitation-promoting region, or a denuded zone change inaccordance with the growing conditions in the case where an 6″-diametercrystal was grown at such a growing rate that the R-OSF position becomesr=⅓R while maintaining such a growing rate almost constant during thebody forming step in a hot zone of a growing furnace where theconventional in-plane temperature distribution within the crystal isimproved so that the amounts of introduced point defects become uniform,and in which the conventional growing conditions are changed so that thecrystal is slowly cooled while staying in the temperature range of 1100°C.-1000° C.

After stabilizing the melt within the crucible under the same conditionsas in Example 5, the power of the heater was adjusted when the singlecrystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatthe pulling rate is high at the initial stage and thus R-OSF occursaround the outer edge. When pulled to a length of 100 mm, the singlecrystal was grown at such a pulling rate that the in-plane R-OSFposition is r=⅓R, and examinations were then made as to the behaviors ofR-OSF, the denuded zone, and the FPD formed in the inner region of R-OSFat different sites of the crystal.

The form of R-OSF and how grown-in defects occur were examined bycoating with Cu onto a wafer processed from the as-grown crystal thatwas grown in accordance with Example 6, heat-treating it at 900° C. tothereby render visible its defective regions by type, and thereaftertaking X-ray topographic pictures. It was found out that the R-OSF widthwas narrower but the oxygen precipitation-promoting region or thedenuded zone was greatly expanded, compared with the conventionalcrystal. Further, it was also verified that even if the R-OSF regionmoved inward, no dislocation clusters occurred.

The FPD distribution density in the as-grown crystal that was grown inaccordance with Example 6 was similar to that of FIG. 9 describedpreviously. Neither FPD nor dislocation clusters were observed when theR-OSF position was r=⅓R

FIG. 15 shows the results of examinations made as to time-zerodielectric breakdown (TZDB) of the wafers processed from the singlecrystal that was produced in accordance with Example 6. The percentnondefective in terms of TZDB in the in-plane crystal area was 95% ormore in the case where the thickness of an oxide film was 25 nm, theapplied voltage was 8 M/V, the R-OSF position was r=⅓R and the FPDdensity was very small.

2-3. EXAMPLE 7

An 8″-diameter single crystal was produced using the single crystalproducing apparatus shown in FIG. 1. The crucible was charged with 120kg of polysilicon that is a raw material for the preparation of acrystal, and boron was further added as a p-type dopant so as to obtainan electrical resistivity of 10 Ωcm. After achieving an Ar atmospherewithin the chamber, the power of the heater was adjusted so as to meltall the raw material for the crystal. After stabilizing the melt withinthe crucible, the lower end of a seed crystal was immersed into themelt, and the single crystal was pulled while rotating the crucible andthe pulling shaft.

In Example 7, it is an object to examine how the widths of R-OSF and adenuded zone change, or how the FPD density changes in the case wherethe conventional growing conditions are changed so that a crystal israpidly cooled while staying in the temperature range of 980° C.-900° C.To do so, pulling rate changing tests were conducted by improving theconventional temperature distribution in an in-plane crystal area sothat the amounts of vacancies introduced into the in-plane crystal areabecome uniform, and by growing the single crystal in a hot zone wherethe crystal can be rapidly cooled while staying in a predeterminedtemperature range.

The form of R-OSF was examined by splitting the as-grown crystal thatwas grown in accordance with Example 7 lengthwise, coating the splitcrystal with Cu, heat-treating it at 900° C. to thereby render visibleits defective regions by type, and thereafter taking X-ray topographicpictures. Compared with the conventional crystal, the R-OSF width andthe denuded zone were greatly expanded. The R-OSF width fluctuated froma maximum of 40 mm to about 4 mm depending on the single crystal length.Even if R-OSF did occur in the in-plane crystal area, the outer regionof R-OSF similarly expanded, and thus no dislocation clusters occurred.Further, even when R-OSF disappeared into the in-plane crystal area, nodislocation clusters occurred. That is, the silicon single crystal waferof this invention can expand regions where no grown-in defects areobserved into the in-plane crystal area by controlling where the outsidediameter or the inside diameter of R-OSF is located.

The relationship between the in-plane R-OSF position and the FPDdistribution density in the as-grown crystal that was grown inaccordance with Example 7 was similar to that shown in FIG. 10 describedpreviously. That is, it is understood that FPD were observed around thecenter of the crystal when the R-OSF position was r=⅖R, and no FPD wereobserved when r=⅓R or less. Therefore, by controlling the in-planeposition of the outside diameter or the inside diameter of R-OSF whileadjusting the growing conditions, one can grow a crystal in the in-planearea of which laser scattering tomography defects (FPD) are present at amarkedly smaller density or not observed, and grown-in defects such asdislocation clusters are not observed.

When examinations were made as to time-zero dielectric breakdown (TZDB)of the wafers processed from the single crystal that was produced inaccordance with Example 7, their results were similar to those of FIG.15 described previously. The percent nondefective in terms of TZDB inthe in-plane crystal area was 95% or more in the case where thethickness of an oxide film was 25 nm, the applied voltage was 8 M/V, theR-OSF position was r=⅓R, and the FPD density was very small.

2-4. EXAMPLE 8

In Example 8, examinations were made as to how R-OSF and a denuded zonechange and how the FPD density changes in accordance with the growingconditions by growing an 8″-diameter single crystal at such a pullingrate that the in-plane R-OSF position is r=⅓R. To do so, the crystal wasgrown in a hot zone of the growing furnace where the conventionalin-plane temperature distribution within the crystal is improved so thatthe amounts of introduced point defects become uniform and in which theconventional growing conditions are changed so that the crystal israpidly cooled while staying in the temperature range of 980° C.-900° C.

After stabilizing the melt within the crucible under the same conditionsas in Example 7, the power of the heater was adjusted when the singlecrystal growing process shifted to the body-growing step via theseed-constricting step and the shoulder-forming step, and the singlecrystal was pulled by a predetermined length under such a condition thatthe pulling rate is high at the initial stage and thus R-OSF occursaround the outer edge. When pulled to a length of 100 mm, the singlecrystal was grown to a length of 1000 mm at such a pulling rate that thein-plane R-OSF position is r=⅓R, and examinations were then made as tothe behaviors of R-OSF, the denuded zone, and the FPD formed in theinner region of R-OSF at different sites of the crystal.

A wafer processed from the as-grown crystal that was grown in accordancewith Example 8 was coated with Cu, heat-treated at 900° C. to therebyrender visible its defective regions by type, and thereafter had X-raytopographic pictures taken. When comparisons were made with theconventional crystal, it was verified that the R-OSF width was narrowerbut the denuded zone was greatly expanded and that no dislocationclusters were formed even if R-OSF occurred in the in-plane crystalarea.

The FPD distribution density in the as-grown crystal that was grown inaccordance with Example 8 was similar to that of FIG. 12 describedpreviously. When the R-OSF position was r=⅓R, no FPD were observedinside R-OSF and no dislocation clusters were observed outside R-OSF.Therefore, by adjusting the growing conditions, one can obtain a crystalin the in-plane area of which the densities of laser scatteringtomography defects (FPD, COP) and dislocation clusters are extremelysmall.

Examinations were also made as to time-zero dielectric breakdown (TZDB)of the wafers processed from the single crystal that was produced inaccordance with Example 8. Their results were similar to those inExample 7. That is, the percent nondefective in terms of TZDB in thein-plane crystal area was 95% or more in the case where the thickness ofan oxide film was 25 nm, the applied voltage was 8 M/V, the R-OSFposition was r=⅓R, and the FPD density was very small.

As described in the foregoing, according to the second high-qualitysilicon single crystal of this invention, the position where R-OSFoccurs can be controlled and regions denuded of laser scatteringtomography defects and dislocation clusters, which are grown-in defects,can be expanded, independently of the width of R-OSF occurring in itsin-plane area as well as even when R-OSF do not appear conspicuously dueto low oxygen contents of the crystal. Therefore, a semiconductingmaterial excellent in device characteristics can be suppliedefficiently.

3. Third High-Quality Silicon Single Crystal and Method of Producing theSame

In inventing third high-quality silicon single crystal wafers, theinventors made examinations as to how dislocation clusters occur inrelation to the position and width of R-OSF occurring in a singlecrystal wafer that was grown under the conventional conditions.

According to the results of the above-described examinations, under theconventional growing conditions, dislocation clusters begin to grow inthe outer region of R-OSF when the in-plane R-OSF position is r=⅔R, anddislocation clusters usually exist when r=½R or less. By the way, whenthe inner region of R-OSF narrows as r=½R or less, the density of laserscattering tomography defects occurring in the inner region can bedecreased, and thus time-zero dielectric breakdown (TZDB) can beimproved. If the formation of dislocation clusters can be suppressed atthe same time, not only time-zero dielectric breakdown (TZDB) but alsodevice characteristics can be improved.

As a result of further investigations made based on the above-describedviewpoint, the inventors were successful in decreasing the density ofgrown-in defects in the whole in-plane crystal area by expanding adenuded zone free of dislocation clusters in the outer region of R-OSFcompared with the single crystal wafer grown under the conventionalconditions. Specifically, when a single crystal was grown based on theCZ method, the formation of dislocation clusters was suppressed bycontrolling the temperature distribution in the single crystal while thecrystal stays in the temperature range of the solidifying interfacetemperature (melting point) to 1250° C. and the heat history of thesingle crystal in the temperature range of 1250° C.-1000° C. so thatconcentrations of vacancies and interstitial silicon (Si) atomsintroduced into an in-plane area of the crystal become equal and uniformin the in-plane area as much as possible.

That is, finding out that laser scattering tomography defects occur in aregion where the vacancy concentration exceeds the interstitial Si atomconcentration, and dislocation clusters occur in a region where theinterstitial Si atom concentration exceeds the vacancy concentration,the formation of grown-in defects was suppressed by minimizingdifferences between the vacancy concentration and the interstitial Siatom concentration in the in-plane crystal area of a wafer.

The third high-quality silicon single crystal of this invention has beenaccomplished on the basis of such findings. It is a silicon singlecrystal grown under such a condition that the crystal stays in atemperature range of 1250° C.-1000° C. for 7 hours or more when pulledby a CZ method, and is characterized in that any of “the outsidediameter of R-OSF,” “the inside diameter or the outside diameter of anoxygen precipitation-promoting region,” “the outside diameter of aring-like region where the amount of oxygen precipitates is small,” and“the outside diameter of a circular region where laser scatteringtomography defects are detected” is within a range of 0-60% of thediameter of the grown crystal.

In the third high-quality silicon single crystal, the reason why “theinside diameter or the outside diameter of an oxygenprecipitation-promoting region” and “the outside diameter of a circularregion where laser scattering tomography defects are detected” are usedas references of the R-OSF position, in addition to “the outsidediameter of R-OSF,” is as follows. Since fluctuations in the width ofR-OSF occurring in an in-plane area of a wafer and irregular behaviorsof R-OSF appearing at one time and disappearing at another depend on theheat history of the crystal during growth, the R-OSF position iscontrolled by getting rid of these factors. Further, the reason why “theoutside diameter of R-OSF” is used as a reference is because where itsoutside diameter is located remains unchanged even if the R-OSF widthchanges in dependence on the heat history. The reason why “the insidediameter or the outside diameter of an oxygen precipitation-promotingregion” is used as a reference is because considerations are given forcases where the inside diameter of the oxygen precipitation promotingregion cannot be used as a reference due to the fact that the insidediameter of the oxygen precipitation promoting region has disappearedinto the in-plane crystal area. Further, the reason why “the outsidediameter of a ring-like region where the amount of oxygen precipitatesis small” is used as a reference is because considerations are given forcases where R-OSF may not appear conspicuously as single crystals tendto contain less oxygen.

Furthermore, the reason why the R-OSF position is limited within therange of 0-60% of the diameter of the grown crystal is because withinsuch a range, grown-in defects can be reduced to an extremely smalllevel or eliminated. Usually, the growing rate is controlled so thatR-OSF is located at a predetermined position with respect to thediameter of the crystal. To effect proper growing rate control, growingrate changing tests are conducted in accordance with a predeterminedsingle crystal producing apparatus and growing conditions so that therelationship between the growing rate and the R-OSF positioncorresponding to FIG. 2 described previously can be grasped in advance.

In the third high-quality silicon single crystal, it is required thatthe heat history of the crystal while the crystal stays in thetemperature range of 1250° C.-1000° C. be controlled during growth basedon the CZ method so that concentrations of vacancies and interstitialsilicon (Si) atoms introduced into an in-plane crystal area become asuniform as possible over the whole in-plane area. At that time, thetemperature distribution of the single crystal being grown influencesthe concentration of vacancies introduced into the in-plane crystalarea.

FIG. 16 illustrates the temperature distribution of a single crystalbeing grown and the behavior of vacancies resulting from suchtemperature distribution. FIG. 16(a) shows how a single crystal is beinggrown in the case where temperature gradients in the axial direction arelarge, and FIG. 16(b) how a single crystal is being grown in the casewhere the same temperature gradients are small. In the single crystalshown in (a) has large temperature gradients in the axial direction, andthus temperature drops are larger toward the outer edge of the crystal,and isotherms on the single crystal exhibit larger temperature gradientstoward the outer edge. In contrast, the single crystal shown in (b) hassmall temperature gradients in the axial direction, and thus temperaturegradients in the axial direction are small at the outer edge of thecrystal with isotherms being either flat ideally, or convex toward themelt (hereinafter referred to simply as “downwardly convex”).

Under large temperature gradients as shown in FIG. 16(a), vacancies thatare introduced into the in-plane crystal area disappear in large amountsdue to up-hil diffusion in which an axial diffusion of vacancies occurtoward the solid-melt interface, and thus the concentration of vacanciesheld in the in-plane crystal area become small. In this case,concentrations of vacancies that are introduced into the in-planecrystal area are not the same due to the fact that temperature gradientsin the axial direction differ in the radial direction. Thus, vacancyconcentrations are lower toward the outer edge where there are markedtemperature drops. On the other hand, under small temperature gradientsas shown in FIG. 16(b), vacancy concentrations in the in-plane crystalarea become uniform, and their concentration distribution in the radialdirection is also stabilized.

FIG. 17 shows how the concentration distributions of vacancies andinterstitial Si atoms in an in-plane area of a single crystal changenear the solid-melt interface and in the range of the melting point to1250° C. during single crystal growth. FIG. 17(a) shows theconcentration distributions near the solid-melt interface; and FIGS.17(b) and(c) show how the concentration distributions change in thetemperature range of the melting point to 1250° C. In each drawing, thevacancy concentration is indicated by C_(V) and the interstitial Si atomconcentration by C_(I).

As shown in FIG. 17(a), first, vacancies and interstitial Si atoms areintroduced into an in-plane crystal area near the interface betweensolid and melt. In this case, both vacancies and interstitial Si atomsare introduced at thermal equilibrium concentrations at the solid-meltinterface. Since the thermal equilibrium concentration of vacanciesexceeds that of interstitial Si atoms, the vacancy concentration exceedsthe interstitial Si atom concentration.

Next, when the temperature of the single crystal being grown is in therange of the melting point to 1250° C., the up-hil diffusion ofvacancies in the axial direction and their concentration gradientdiffusion in the radial direction are promoted, as shown in FIG. 17(b),because the diffusion coefficient of vacancies exceeds that ofinterstitial Si atoms. Further, vacancies diffuse more markedly in bothaxial and radial directions with decreasing growing rate. As a result,concentrations of vacancies in the in-plane crystal area are decreaseddue to their disappearance caused by their axial up-hil diffusion towardthe solid-melt interface, and their concentrations are further decreasedat the outer edge of the crystal by their concentration gradientdiffusion in the radial direction. On the other hand, interstitial Siatoms remain in the in-plane crystal area and their concentrations donot drop so much at the outer edge of the crystal in the temperaturerange of the melting point to 1250° C. because their diffusioncoefficient is smaller than the diffusion coefficient of vacancies andthus their axial up-hil diffusion occurs only to a small degree in suchtemperature range.

Therefore, when the growing rate is kept low, the vacancy concentrationexceeds the interstitial Si atom concentration in the inner side of thecrystal, and the latter may exceed the former at the outer edge of thecrystal in some cases as shown in FIG. 17(c). From the previouslydescribed presumption that dislocation clusters would be an agglomerateof excessive interstitial Si atoms, drawn is a conclusion thatdislocation clusters are formed in the outer edge region of the crystalwhere the interstitial Si atom concentration exceeds the vacancyconcentration as described above as the crystal is gradually cooled(about 1000° C.).

Further, when the single crystal temperature is in the range of 1250°C.-1000° C., particularly, near 1250° C., as the growing processproceeds, the diffusion coefficient of interstitial Si atoms exceedsthat of vacancies. Thus, by allowing the crystal to stay in thistemperature range for a prolonged period, concentration gradientdiffusion of interstitial Si atoms can be promoted at the outer edge ofthe crystal. Therefore, even if the interstitial Si atom concentrationexceeds the vacancy concentration at the outer edge of the crystal asshown in FIG. 17(c) described above, if the single crystal is slowlycooled while it stays in the temperature range of 1250° C.-1000° C.,diffusion of interstitial Si atoms in the radial direction can beencouraged to thereby decrease their concentrations, and thus the regionwhere interstitial Si atoms are excessively present can be reduced. As aresult, the formation of dislocation clusters in the outer edge regionof the crystal can be suppressed.

In the third high-quality silicon single crystal, it is required thatthe crystal stay in the temperature range of 1250° C.-1000° C. for 7hours or more while it is being pulled. In Example 9, which will bedescribed later, at a site of a crystal that is slowly cooled under sucha condition the crystal stays in the target high temperature range for 8hours, the formation of dislocation clusters is suppressed, and adenuded zone outside R-OSF is greatly expanded. Further, it has beenverified from the results of various tests that the formation ofdislocation clusters is eliminated by causing the crystal to stay in thetemperature range of 1250° C.-1000° C. for 7 hours or more.

It is as described earlier that in an in-plane crystal area of a wafer,laser scattering tomography defects occur in the region where thevacancy concentration exceeds the interstitial Si atom concentration andthat dislocation clusters occur in the region where the interstitial Siatom concentration exceeds the vacancy concentration. By the way, in anin-plane crystal region where differences between the vacancyconcentration and the interstitial Si atom concentration are small, bothtypes of point defects are re-combined and disappear, so that grown-indefects are no longer formed in such crystal in-plane region. In orderto form such a region where differences between the vacancyconcentration and the interstitial Si atom concentration are small, itis required that concentrations of vacancies introduced into thein-plane crystal area be not only uniform but also approximated to theconcentration distribution of interstitial Si atoms.

To achieve the above-described concentration distribution, it isrequired that the temperature gradient in the axial direction be smallat the outer edge by making the isotherms of a single crystal flat ordownwardly convex while the crystal stays in the temperature range ofthe solidifying interface temperature (melting point) to 1250° C., asshown in FIG. 16(b) described previously. As a result of thisarrangement, the concentration distribution of vacancies held in thein-plane crystal area becomes uniform in the whole in-plane area due totheir up-hil diffusion occurring in this temperature range. On the otherhand, axial up-hil diffusion of interstitial Si atoms occurs only to asmall degree and their concentrations do not drop so much at the outeredge because their diffusion coefficient is smaller than the diffusioncoefficient of vacancies in the temperature range of the solidifyinginterface temperature (melting point) to 1250° C., and thus differencesbetween the vacancy concentration and the interstitial Si atomconcentration can be decreased. Therefore, in growing a silicon singlecrystal wafer of this invention, it is desirable to make the isothermsin the single crystal flat or downwardly convex in the temperature rangeof the solidifying interface temperature (melting point) to 1250° C.

In the silicon single crystal wafer of this invention, the formation ofdislocation clusters appearing outside R-OSF can be suppressed and thusregions free of grown-in defects can be expanded by improving thegrowing conditions as described above. As a result, regions where laserscattering tomography defects and dislocation clusters, whichdeteriorate device characteristics, occur can be driven out of thein-plane area of the wafer, and thus one can obtain high-quality siliconsingle crystal wafers that can exhibit excellent characteristics.

To evaluate the third high-quality silicon single crystal, 8″-diametersilicon single crystals were produced, and the form of R-OSF and howcrystal defects occur were examined based on Examples 9 to 12.

3-1. EXAMPLE 9

An 8″-diameter single crystal was produced using the single crystalproducing apparatus shown in FIG. 1. The crucible was charged with 120kg of polysilicon that is a raw material for the preparation of acrystal, and boron was further added as a p-type dopant so as to obtainan electrical resistivity of 10 Ωcm. After achieving an Ar atmospherewithin the chamber, the power of the heater was adjusted so as to meltall the raw material for the crystal. After stabilizing the melt withinthe crucible, the lower end of a seed crystal was immersed into themelt, and the single crystal was pulled while rotating the crucible andthe pulling shaft.

In Example 9, in order to examine how the behavior of dislocationclusters in their formation changes in accordance with the growingconditions, the crystal was grown to a body length of 500 mm at apredetermined growing rate under such a condition that R-OSF is locatedat r=⅖R and thus dislocation clusters are located in the outer region ofR-OSF. When the single crystal pulling process shifted to thebody-growing step via the seed-constricting step and theshoulder-forming step, the single crystal was pulled by a predeterminedlength while adjusting the pulling rate and the power of the heater sothat the diameter of the crystal can be maintained.

When the single crystal was pulled to a length of 500 mm, the crystalgrowing process was stopped for a predetermined time interval, and thecrystal was slowly cooled in each of the temperature ranges duringgrowth. Thereafter, the crystal growing process was resumed, and shiftedto the tail-constricting step when the single crystal was pulled to alength of 1000 mm. Through these process steps, how these process stepsaffect the behavior of dislocation clusters in their formation wasexamined. For purposes of comparison, a single crystal that was grown ata constant pulling rate without stoppage was also prepared as acomparative example.

FIG. 18 schematically shows the results of observations made throughX-ray topography as to the single crystals pulled in Example 9 aftersplitting the single crystals in the as-grown state lengthwise, coatingthe split crystals with Cu, and heat-treating them at 900° C. to therebyrender visible their defective regions by type. FIG. 18(a) indicates thecomparative example, and FIG. 18(b) the example of the inventionobtained through 8 hours of stoppage. In the example of the invention,it is understood that the formation of dislocation clusters wassuppressed at the site that was slowly cooled in the temperature rangeof 1200° C.-1050° C. and that the denuded zone outside R-OSF was greatlyexpanded, compared with the comparative example.

According to the examination results, the more prolonged the stoppage ofthe crystal growth, the wider the width across which the formation ofdislocation clusters is suppressed, and thus the area where the denudedzone occupies is increased. By controlling the growing conditions inthis way, the density at which dislocation clusters occur in thein-plane crystal area can be markedly reduced.

3-2. EXAMPLE 10

In Example 10, how the width across which dislocation clusters areformed changes was examined in the case where an 8″-diameter crystal wasgrown at an almost constant pulling rate using an improved hot zonewhere R-OSF is located at r=⅖R and the temperature of the crystal canstay in the range of 1250° C.-1000° C. for 10 hours. To do so, thecrystal was pulled to a length of 100 mm while adjusting the pullingrate and the power of the heater so that the diameter of the crystal ismaintained under the same conditions as in Example 9. Then, whilemaintaining the pulling rate constant, the single crystal was pulled toa length of 1000 mm, after which the process shifted to thetail-constricting step.

FIG. 19 schematically shows the results of observations made throughX-ray topography as to the single crystal pulled in Example 10 aftersplitting the single crystal in the as-grown state lengthwise, coatingthe split crystal with Cu, and heat-treating it at 900° C. to therebyrender visible its defective regions by type. It is understood thatdislocation clusters outside R-OSF disappeared and the denuded zone wasgreatly expanded, compared with the previously described comparativeexample.

FIG. 20 shows the results of observations made through Secco etching asto the FPD distribution density in the as-grown crystal that was grownin accordance with Example 10. In the conventional growing method inwhich the crystal is not slowly cooled while staying in the temperaturerange of 1250° C.-1000° C., dislocation clusters usually occur outsideR-OSF. In contrast, under the growing conditions of Example 10, thecrystal free of dislocation clusters in its in-plane area can beobtained. On the other hand, it is understood that the FPD density canbe reduced in the inner region of R-OSF in accordance with the growingconditions of Example 10.

FIG. 21 shows the results of measurements made through infraredscattering tomography as to the distribution density of laser scatteringtomography defects in the as-grown crystal that was grown in accordancewith Example 10. It is apparent from the results shown in FIG. 20 thatthe R-OSF position can be grasped by measurements through infraredscattering tomography.

3-3. EXAMPLE 11

In Example 11, how the formation of laser scattering tomography defects,R-OSF or a denuded zone changes was examined in the case where an8″-diameter crystal was grown at an almost constant pulling rate undersuch a condition that the R-OSF position is r=¼R, using an improved hotzone where a uniform temperature distribution is achieved within thecrystal in the temperature range of the melting point to 1250° C. sothat the amounts of introduced vacancies become uniform and in which thecrystal can stay in the temperature range of 1250° C.-1000° C. for 10hours. The crystal was pulled to a length of 100 mm while adjusting thepulling rate and the power of the heater so that the diameter of thecrystal is maintained under the same conditions as in Example 9. Then,while maintaining the pulling rate constant, the single crystal waspulled to a length of 1000 mm, after which the process shifted to thetail-constricting step.

FIG. 22 schematically shows the results of observations made throughX-ray topography as to the single crystal pulled in Example 11 after thesingle crystal was sliced in the as-grown state, coated with Cu, andheat-treated at 900° C. to thereby render visible its defective regionsby type. No dislocation clusters occurred and the denuded zone wasgreatly expanded despite the fact that R-OSF did occur at r={fraction(1/4)}R in the inner side of the in-plane crystal area. Further, whenthe R-OSF position was r=¼R, no FPD derived from laser scatteringtomography defects were observed even in the inner region of R-OSF.Therefore, it is understood that the densities of laser scatteringtomography defects (FPD, COP) and dislocation clusters can be reduced inthe in-plane crystal area by controlling the growing conditions.

FIG. 23 shows the results of examinations made as to time-zerodielectric breakdown (TZDB) of the wafers processed from the singlecrystal that was pulled in Example 11. The percent nondefective in termsof TZDB in the in-plane crystal area was 95% or more in the case wherethe thickness of an oxide film was 25 nm, the applied voltage was 8 M/V,the position of R-OSF was r=¼R, and the FPD density was extremely small.

3-4. EXAMPLE 12

In Example 12, how the formation of laser scattering tomography defects,R-OSF or a denuded zone changes was examined in the case where an8″-diameter crystal was grown at an almost constant pulling rate undersuch a condition that R-OSF die out from the in-plane crystal area,using an improved hot zone where a uniform in-plane temperaturedistribution within the crystal is achieved in the temperature range ofthe melting point to 1250° C. so that the amounts of vacanciesintroduced become uniform and where the crystal can stay in thetemperature range of 1250° C.-1000° C. for 10 hours. The crystal waspulled to a length of 100 mm while adjusting the pulling rate and thepower of the heater so that the diameter of the crystal is maintainedunder the same conditions as in Example 9. Then, while maintaining thepulling rate constant, the single crystal was pulled to a length of 1000mm, after which the process shifted to the tail-constricting step.

FIG. 24 schematically shows the results of observations made throughX-ray topography as to the single crystal pulled in Example 12 after thecrystal was sliced in the as-grown state, coated with Cu, andheat-treated at 900° C. to thereby render visible its defective regionsby type. R-OSF died out into the center of the in-plane crystal area.Although the oxygen precipitation-promoting region appeared, nodislocation clusters occurred and the denuded zone was greatly expanded.Further, no FPD derived from laser scattering tomography defects wereobserved by the dying out of R-OSF. Therefore, controlling the growingconditions can reduce the density of grown-in defects.

In Example 12, another crystal was grown at such a pulling rate that theoxygen precipitation-promoting region dies out from the in-plane area ofthe crystal. In this case, although not shown in the drawing, thedenuded zone occupied the whole in-plane crystal area, from which theoxygen precipitation-promoting region was extinct.

From examinations made as to time-zero dielectric breakdown (TZDB) ofwafers processed from the single crystals that were pulled in Example12, results similar to those of Example 3 were obtained. That is, thepercent nondefective in terms of TZDB in the in-plane crystal area was95% or more in the wafers where R-OSF were extinct inward under theconditions that the thickness of an oxide film was 25 nm and the appliedvoltage was 8 M/V.

As described in the foregoing, according to the third high-qualitysilicon single crystal and the method of producing the same of thisinvention, regions free of laser scattering tomography defects anddislocation clusters, which are grown-in defects, can be expanded intothe in-plane area of a wafer by controlling the R-OSF position whileadjusting the single crystal growing conditions. Therefore, asemiconducting material excellent in device characteristics can besupplied.

4. Fourth High-Quality Silicon Single Crystal and Method of Producingthe Same

As described earlier, it is known as the R-OSF phenomenon that theradius of R-OSF continuously expands outward with increasing pullingrate and shrinks inward to disappear with decreasing pulling rate duringsingle crystal growth. Here, the R-OSF distribution that is shown infunction of the pulling rate in FIG. 2 described previously is V-shaped,and the denuded zones are present so as to be inscribed in R-OSF andclosely outside R-OSF in narrow ranges, respectively, as shown in FIG. 3described previously. Now, if one can find such growing conditions thatthe V-shaped form opens as wide as possible upward or becomes flat ifpossible and that the R-OSF so re-shaped is located at an appropriateposition in a wafer, then a single crystal capable of supplying waferscontaining minimal defects must be produced. It is assumed that theR-OSF position is greatly affected by the cooling rate aftersolidification or the temperature gradients in the direction of thepulling shaft, in addition to the above-described pulling rate.

On the basis of these assumptions, in inventing a fourth high-qualitysilicon single crystal, attention was paid to positively changing thetemperature distribution within a solidified single crystal. That is,the single crystal being pulled should be cooled not simply naturallybut by controlling the cooling conditions. However, since it isdifficult to actually measure the temperature distribution within thesingle crystal being pulled, it is calculated by a heat transferanalysis simulation method.

FIG. 25 schematically illustrates the temperature distribution within asingle crystal that is being pulled. Since the single crystal beingpulled is usually cooled from its surface, there are larger temperaturedrops at the surface than at the inner side as shown in FIG. 25(a). Thatis, assuming a surface of a wafer that is horizontal with respect to avertical pulling shaft, its temperature is high at the center and low atthe outer edge. If the solidification process proceeds at a horizontalplane that is substantially the same as the melt surface, then thetemperature of the crystal on the horizontal plane immediately aftersolidification must be the same at the center and the outer edge.Therefore, assuming a temperature distribution in the vertical directionparallel with the pulling shaft at a position slightly distant from thesolid-melt interface of a single crystal that is being grown, thetemperature gradient is larger at the outer edge than at the center.

In contrast, by growing a single crystal under various conditions whilechanging the method of cooling its surface during pulling so that itsinside temperature distribution is different from that which is usual,the distribution of defects in wafers obtained from this single crystalwas examined. The examinations revealed the following.

(i) When the temperature distribution within a single crystal is changedin the course of cooling the single crystal while the crystaltemperature at the center stays from the temperature immediately aftersolidification (1412° C.) to 1250° C., the R-OSF width can be expandedeven if the outside diameter of R-OSF remains the same.

(ii) When the R-OSF width expands, so do the oxygen precipitationpromoting region and the denuded zone immediately outside R-OSF.

(iii) The temperature distribution during pulling that contributes toexpanding the R-OSF width must satisfy such a requirement that thetemperature gradient in the vertical direction parallel with the pullingshaft of a single crystal be smaller at the outer edge than at thecenter; i.e., as shown in FIG. 25(b), the temperature must be higher atthe outer edge than at the center on a horizontal plane in the crystalor in a surface of a wafer.

(iv) To make the temperature gradient smaller at the outer edge than atthe center of a crystal, the hot zone, i.e., the method of cooling asingle crystal portion being pulled must be changed. To obtain a singlecrystal having less defects, the temperature gradients in the verticaldirection of the whole single crystal must be made larger than those ofthe conventional example, and hence the pulling rate must be increased.

Although the presence of R-OSF is not necessarily so important a factorin view of the trends toward lower temperature fabrication of devicesand lower oxygen content in crystals as described earlier, to know thelocation of R-OSF would give a guidance for determining the growingconditions of a single crystal. From this viewpoint, the inventorsdecided to select such growing conditions as to expand a denuded zone asmuch as possible from the relationship between the R-OSF position andthe temperature distribution within a crystal that is being pulled. Inthis case, since the R-OSF width changes, the detectable outsidediameter of R-OSF in each of wafers sliced from obtained single crystalswas measured. Through these measurements, the inventors were able toclarify the influence of the outside diameter of R-OSF and thetemperature distribution within a single crystal being pulled upon thedistribution of defects, and thus accomplished the fourth high-qualitysilicon single crystal.

That is, a method of producing the fourth high-quality silicon singlecrystal is characterized in that a silicon single crystal is grown undersuch conditions that a temperature gradient in the vertical directionparallel with a pulling shaft of the crystal is smaller at the outeredge than at the center and is 2.6° C./mm or more at the center when thesingle crystal stays in a temperature range of its solidifying point to1250° C. during growth, and that the outside diameter of R-OSF is withina range of 0-60% of the diameter of the grown crystal.

Why does a denuded zone expand when the temperature distribution duringcooling is changed? The following answers this question. First, the meltundergoes a change to a solid crystal through solidification when pulledat the time of single crystal growth. Since such a change takes placefrom a liquid phase in which atoms are randomly arranged to a solidphase in which atoms are neatly arranged, in the solid phase near thesolid-melt interface, there are vacancies created by atoms that shouldexit missing and interstitial atoms created by superfluous Si atomsentering a crystal lattice in large amounts. Since a liquid in whichinter-atomic distance is large undergoes a change in state to a solid,it is presumed that there would be more vacancies created by missingatoms than interstitial atoms in the crystal immediately aftersolidification. As the portion that is grown into a single crystal aftersolidified by pulling moves away from the solid-melt interface, atomsand vacancies move and diffuse or vacancies and interstitial atoms arecombined together, etc., so that these vacancies and interstitial atomsdisappear one after another, causing atoms in the crystal to be neatlyarranged. However, some of these vacancies and interstitial atoms doremain in the crystal.

Of the vacancies and interstitial atoms introduced into the crystal inthe solidification process, the former outnumbers the latter. Both canmove around within the crystal quite freely while the crystaltemperature is high, with vacancies moving or diffusing faster thaninterstitial atoms. And it is assumed that the number of these vacanciesand interstitial atoms is reduced while disappearing in the course oftheir diffusion, their diffusion to the surface, their combination andthe like, which are based chiefly on the temperature gradient.

First, the saturation limit concentrations of vacancies and interstitialatoms in a high-temperature crystal become smaller at lowertemperatures, respectively. Thus, even if both of them are present inequal amounts, one at lower temperatures has higher concentrations andthe other at higher temperatures has lower concentrations as the actualeffect. A single crystal that is being grown has temperature gradientsin the vertical direction, and actual concentration differencesattributable to these temperature gradients cause diffusion to occurfrom the low-temperature side to the high-temperature side, i.e., fromthe upper side of a single crystal being grown to the solid-meltinterface, and thus the number of vacancies and interstitial atomsdecreases with decreasing temperature. Further, since vacancies arecreated when atoms constituting a crystal lattice are missing andinterstitial atoms are created when superfluous atoms are present in thelattice, when both of them collide, they are combined with each other todisappear, making the crystal lattice perfect.

The temperature gradients in the direction of the vertical axis of acrystal being grown little change even if the pulling rate is changed.That is, at the same temperature gradient, vacancies diffuse toward thesolid-melt interface in the same amount per unit time. Hence, when thepulling rate is increased, the crystal temperature decreases, leavingexcessive vacancies undiffused, and even if vacancies disappear at anincreased pace through their diffusion to the surface and theircombination with interstitial atoms, the undiffused vacancies do remainin the crystal as defects, and it is assumed that these undiffusedvacancies would be the cause of laser scattering tomography defects. Onthe other hand, when the pulling rate is decreased, vacancies diffuseand disappear sufficiently, but interstitial atoms, which diffuse moreslowly than vacancies, are left undiffused while the crystal temperatureis decreasing, and thus these undiffused interstitial atoms causedislocation clusters to occur. It is hence assumed that these phenomenawould be responsible for the fact shown in previously described FIG. 2that laser scattering tomography defects mainly result when the pullingrate is high and dislocation clusters mainly result when the pullingrate is low.

However, when a single crystal is grown at intermediate pulling rates,for example, as shown in previously described FIG. 3, laser scatteringtomography defects distribute around the center and dislocation clustersnear the outer edge with R-OSF, an oxygen precipitation promotingregion, and denuded zones existing in between. In the case of a normalsingle crystal pulling and growing method, the temperature gradient inthe direction of the vertical axis is larger at the surface than at thecenter of the single crystal as shown in FIG. 25(a). This means thattemperature gradient-based diffusion takes place faster at the surfacethan at the center, and since vacancies diffuse faster, theconcentration of interstitial atoms becomes relatively higher withdecreasing pulling rate, and hence dislocation clusters begin to appear.At this point in the process, a relatively large amount of vacanciesstill exists around the center where the temperature gradients aresmall, and thus they remain as laser scattering tomography defects. Inthe region between them, vacancies balance with interstitial atoms innumber, allowing them to be combined together to rid the crystal ofsources of defects. Thus, it is assumed that this is why a denuded zoneis formed. According to one theory, OSF is formed by the nucleation ofoxygen precipitates, and the fact that the oxygen precipitationpromoting region circumscribes R-OSF supports this theory.

If a denuded zone is formed at a position where vacancies balance withinterstitial atoms in number during a period in which vacancies andinterstitial atoms can move easily, i.e., while the crystal temperatureis high and thus they can diffuse fast, then the position at whichvacancies balance with interstitial atoms moves outward when the pullingrate is high due to an insufficient reduction of diffusing vacancies,and the same position nears the center when the pulling rate is low dueto the decreased number of vacancies, and thus a region wheredislocation clusters occur would expand near the outer edge. Here, if itis supposed that oxygen precipitation occurs more easily by consumingvacancies at the place where vacancies slightly outnumber interstitialatoms than at the position where they balance in number, it could resultthat oxygen precipitates and R-OSF occur in a region that is inwardlyadjacent to a denuded zone.

If the formation of a denuded zone is dependent on the fact thatvacancies balance with interstitial atoms in number, and if suchbalancing is governed by the temperature gradients in the verticaldirection of a crystal in a high temperature range immediately aftersolidification as described above, then it is assumed that to expand adenuded zone, the pulling rate may be adjusted so that the temperaturegradient in the direction of the vertical axis of a single crystal beingpulled is the same both at the center and the outer edge. However, inactuality, when exactly the same temperature gradient is achieved in thewhole in-plane crystal area, dislocation clusters tend to appear aroundthe outer edge of a wafer as long as the condition that almost all laserscattering tomography defects around the center disappear is satisfied.Thus, to decrease grown-in defects in the whole wafer, the singlecrystal must be grown under limited conditions. The reason for this isassumed as follows. Since vacancies and interstitial atoms disappearupon reaching the surface of a single crystal, their concentrations arelow near the surface of the single crystal, and thus it is assumed thathorizontal concentration diffusion is taking place from the inside tothe surface. In this case also, since the vacancies diffuse faster thaninterstitial atoms, the concentration of interstitial atoms becomesrelatively high, and thus dislocation clusters tend to occur at thesurface. Therefore, if the temperature gradient in the verticaldirection is made smaller at the surface than at the center, thendisappearing vacancies due to their diffusion to the surface can besaved, and hence the formation of dislocation clusters near the surfacecan be suppressed.

In applying the method of producing the fourth high-quality siliconsingle crystal to the growing of a single crystal, the temperaturedistribution within the single crystal is controlled in such atemperature range that the single crystal is cooled to 1250° C. aftersolidification. The reason why the temperature range is limited to 1250°C. is because when the crystal is cooled to temperatures below this,temperature distribution control is no longer effective in expanding adenuded zone.

In such a temperature range that a single crystal being grown is cooledto 1250° C. after solidification, the temperature gradient in thevertical direction at the center of the single crystal is controlled tobe 2.6° C./mm or more. This is because the R-OSF width is hard toincrease at temperature gradients below 2.6° C./mm and thus regions freeof grown-in defects cannot be expanded. This point is consideredimportant in the sense that giving precedence to the disappearance ofvacancies expands the R-OSF width by temperature gradient-dependentvertical diffusion over their diffusion toward the surface of thecrystal. This temperature gradient is allowed to be large from theviewpoint of suppressing the formation of grown-in defects. However,under large temperature gradients, cooling means must be furtherimproved, and unsatisfactory dislocations occur due to distortionscaused by shrinkages at short distances derived from the improvedcooling, and thus the temperature gradient may actually be increased toabout 6.0° C./mm at most. Note that the temperature gradient isdesirably in a range of 3.5-4.5° C./mm.

Further, in the method of producing the fourth high-quality siliconsingle crystal, the temperature gradient in the vertical directionparallel with the central axis is made smaller at the outer edge than atthe center of a crystal as far as a single crystal portion that iscooled to 1250° C. from the solidifying point during pulling isconcerned. In the case of the normal growth, the temperature gradient islarger at the outer edge than at the center in this temperature rangeduring the pulling of a single crystal. That is, the solid-meltinterface of a single crystal being grown is substantially flush withthe melt surface and thus has the same temperature as the melt surface,and hence at positions vertically equidistant from the melt surface, thecrystal temperature is lower at the outer edge than at the center.

In contrast, in the method of producing the fourth high-quality siliconsingle crystal, the temperature gradient at the outer edge is madesmaller than at the center, and thus at positions vertically equidistantfrom the melt surface, the crystal temperature is higher at the outeredge than at the center. In other words, the temperature gradient in thevertical direction parallel with the pulling shaft of the crystal issmaller at a desired position on a plane orthogonal to the pulling shaftthan at any position along a line connecting the center to that desiredposition. The reason why the temperature gradient in the verticaldirection is made smaller at the outer edge than at the center isbecause doing so can expand the R-OSF width observed in wafers. When thetemperature gradient at the outer edge exceeds that at the center, theR-OSF width cannot be expanded.

The outside diameter of R-OSF detected on a plane perpendicular to thegrowth axis taken out of a single crystal, i.e., on a surface of a waferis set within a range of 0-60% of the diameter of the grown crystal.While such outside diameter of R-OSF changes in accordance with thegrowing rate, the growing rate for obtaining the same outside diameterof R-OSF differs depending on the temperature conditions of a singlecrystal being pulled and the hot zone design for a single crystal beinggrown. In view of this, how the outside diameter of R-OSF changes isexamined experimentally by changing the pulling rate using growingequipment, and a single crystal is grown at such a rate that the outsidediameter falls within the above-described range.

When the pulling rate is so high that the outside diameter of R-OSFexceeds 60%, a region where laser scattering tomography defects occurremains around the center of a single crystal. Further, when the pullingrate is continuously decreased, the outside diameter of R-OSF isgradually reduced, finally, to 0%. If the pulling rate is furtherdecreased from such level that the outside diameter of R-OSF is 0%,dislocation clusters begin to occur. To avoid the above inconvenience, asingle crystal is to be grown at such a pulling rate that the outsidediameter of R-OSF is within the range of 0-60% of its diameter.

To achieve a temperature gradient in the vertical direction of 2.6°C./mm or more at the center of a single crystal and a temperaturegradient that is smaller at the outer edge than at the center during thepulling of a single crystal, the upper portion of the single crystalbeing pulled must be cooled forcibly, not naturally, and its surfacethat is located at a predetermined distance from the melt surface mustbe heat-insulated or heated. By cooling the upper portion, the centralportion of the single crystal immediately after solidification is cooledby heat conduction, while its outer edge portion can have highertemperatures than around its center by heat insulation or heating. Anymethod may be used in forcibly cooling the upper portion of the singlecrystal during pulling. Means of purging a cool ambient gas or nearing acooled object to a site to be cooled, etc. may be applied. For example,if a method in which the upper portion of a single crystal is coveredwith a water-cooled sleeve that is concentric with the single crystal isemployed, and when the lower end of the sleeve is located at apredetermined distance from the melt surface, then the crystal surfaceportion extending from the melt surface to the lower end of the sleeveis heat-insulated by radiation from the melt surface and by heat fromthe heater for heating the crucible. As a result, as far as any ofhorizontal planes that are as high as such heat-insulated crystalsurface portion are concerned, lower temperatures can be achieved at thecenter due to the heat conduction that is based on the cooling of theupper portion of the single crystal.

The temperature distribution within a single crystal in this case isobtained by actual temperature measurements on the surface of the singlecrystal and by calculations based on the heat transfer analysissimulation method. As this heat transfer analysis method, a simulationmethod used for the normal silicon single crystal growth may be applied.In such a case, the design of a hot zone, i.e., a portion to be cooledthat is above the melt surface for achieving the above-describedtemperature distribution within the single crystal is determined, afterwhich data is gathered, e.g., by actually measuring the temperaturewhile inserting a thermocouple into the single crystal that is beinggrown, or by measuring the surface temperature of the single crystalbeing pulled, and the gathered data is corrected. By doing so, a morecorrect temperature distribution can be estimated.

Note that in order to set the outside diameter of R-OSF at 60% or lessby effecting the cooling control for controlling the distribution oftemperature gradients in the vertical direction of the single crystalbeing grown, the pulling rate must be particularly higher than in thenormal growth in which no cooling control is effected. This indicatesthat a single crystal having less defects can be grown at higher pullingrates.

To evaluate the fourth high-quality silicon single crystal, 8″-diametersilicon single crystals were produced and the form of R-OSF and howcrystal defects occur were examined based on Examples 13 and 14.

4-1. EXAMPLE 13

An 8″-diameter single crystal was produced using a single crystalproducing apparatus. The crucible was charged with 120 kg of polysiliconthat is a raw material for the preparation of a crystal, and boron wasfurther added as a p-type dopant so that the crystal has an electricalresistivity of about 10 Ωcm. FIG. 26 schematically shows a cross sectionof the single crystal producing apparatus used. This single crystalproducing apparatus is equipped with a double-walled stainless steelcooling sleeve 7 so that the upper portion of a single crystal 6 pulledthereby can be cooled. The lower end of the sleeve 7 is closed and thusits inside can be water-cooled. The sleeve 7 is disposed on theapparatus so as to be vertically movable coaxial with the central axisof the single crystal to be pulled. The inside diameter of the coolingsleeve 7 is 240 mm with respect to the 8″-diameter silicon singlecrystal.

After evacuating the apparatus to achieve an Ar atmosphere therein andmelting the silicon within the crucible using a heater 2, a seed crystalwas pulled while brought into contact with a melt 3. Then, the growingprocess proceeded to the seed-constrictinng step, the shoulder-formingstep, and to the body-growing step. The cooling sleeve 7 had its lowerend located at a distance of 150 mm from the melt surface. After growingthe single crystal to a predetermined diameter, the current of theheater 2 was adjusted to set the pulling rate at 1.5 mm/min, and thegrowing process was thereafter continued. When the shoulder entered thecooling sleeve 7, it was started to slow down the pulling rate. Sincethe melt 3 within the crucible was reduced as the single crystal 6 grew,the crucible 1 was elevated to maintain the melt surface at the samelevel at all times. While the single crystal was being grown to a bodylength of 800 mm, the pulling rate was successively dropped to a levelof 0.5 mm/min, and the single crystal was grown by an additional lengthof 200 mm at such pulling rate, after which the tail-constricting stepwas performed to terminate the growing process. As to the single crystalbeing pulled between the melting point and 1250° C., the followingtemperature gradients were obtained from the calculations made by heattransfer analysis simulations. The temperature gradient was in the rangeof 3.8-4.0° C./mm at the center of the crystal, and in the range of3.2-3.7° C./mm at the outer edge, and they little changed even when thepulling rate was changed. Examinations were made through X-raytopography as to the defect distribution after splitting the obtainedsingle crystal lengthwise along its central axis, cutting therefrom a1.4 mm-thick slice including the central axis, immersing the slice intoa 16%-by-weight aqueous solution of copper nitrate to thereby deposit Cuthereon, and heating the resultant slice at 900° C. for 20 minutes andthen cooling the heated slice.

FIG. 27 shows the results of the examinations made as to the defectdistribution in Example 13. FIG. 27 schematically shows the defectdistribution in function of the pulling rate during growth. If a waferis taken from a plane perpendicular to the central axis of a normallygrown single crystal, the outside diameter of R-OSF is 60% of thediameter of the crystal when the pulling rate is 0.87 mm/min, and isshorter when the pulling rate is lower. When the crystal was pulled at arate of 0.87 mm/min in Example 13, the inner region of R-OSF remainedaround the center. However, the density of laser scattering tomographydefects in this portion was ⅓ or less compared with the wafer obtainedby the conventional method. At pulling rates that were 0.85 mm/min orless, R-OSF almost disappeared. Further, at pulling rates below 0.79mm/min, dislocation clusters began to occur from the vicinity of theouter edge of the single crystal.

It is understood that a single crystal capable of supplying wafershaving an extremely small level of laser scattering tomography defectsand dislocation clusters can be grown by making the temperature gradientin the vertical direction of the single crystal being grown smaller atthe outer edge than at the center and by controlling the pulling rate.

4-2. EXAMPLE 14

An 8″-diameter silicon single crystal was grown by the single crystalproducing apparatus shown in FIG. 26 and used in Example 13. The coolingsleeve and its location were exactly the same as in Example 13. Thesingle crystal having a body length of 1000 mm was grown from 120 kg ofpolysilicon material under such conditions that the temperature gradientis smaller at the outer edge than at the center as in Example 13 andthat the pulling rate after forming the shoulder is almost constant at0.82-0.83 mm/min. The temperature gradient of the single crystal beingpulled while the crystal stayed in the temperature range of the meltingpoint to 1250° C. was 3.9-4.0° C./mm at the center of the crystal and3.3-3.5° C./mm at the outer edge according to the results of heattransfer analysis simulations.

For purposes of comparison, another 8″-diameter single crystal wasgrown. Although the same single crystal producing apparatus was used,the conventional method was applied and the cooling sleeve 7 was thusremoved this time. The pulling rate was set at 0.47 mm/min so that R-OSFappear around the outer edge similarly to the conventional crystal. Thetemperature gradient of this single crystal being grown while thecrystal stayed in the temperature range of the melting point to 1250° C.was 2.0-2.1° C./mm at the center of the crystal and 1.8-1.9° C./mm atthe outer edge according to the results of heat transfer analysissimulations.

The outside diameter of R-OSF was detected and measured by the sametechnique as in Example 13 as to wafers taken at three positions, i.e.,the upper portion, the middle portion, and the lower portion, from thetwo kinds of single crystals obtained. Further, examinations were madethrough laser scattering tomograpy and Secco etching as to the densityof laser scattering tomography defects and the density of dislocationclusters in specimens taken at three positions, i.e., the center, theposition that is ½ the diameter, and the outer edge from each wafer,respectively. Still further, as to wafers taken at positions adjacent tothose wafers whose defect distributions were examined, time-zerodielectric breakdown (TZDB) for an oxide film thickness of 25 nm wasmeasured and their percent nondefective was obtained after subjectingeach of such wafers to a predetermined heat treatment and the like andthereafter giving it a gate structure of a device.

The results of these examinations are collectively indicated in Table 1.The density of laser scattering tomography defects and that ofdislocation clusters are indicated in terms of the average of themeasurements at the three positions of each wafer. As is apparent fromTable 1, the wafers obtained from the single crystal that was grown inaccordance with the method specified by this invention are of highquality with less grown-in defects such as laser scattering tomographydefects and dislocation clusters and with a higher percent nondefectivein terms of TZDB compared with the wafers obtained from the singlecrystal that was grown by the conventional producing method.

TABLE 1 Average Average Percent density of laser density of nondefectiveTempera- Ratio of scattering dislocation in terms ture outsidetomography clusters of initial gradient in Position diameter defects(number of oxide film vertical in single of R-OSF (number of clusters/withstand direction crystal (%) scatterers/cm³) cm³) voltage RemarksLarge at Upper 0 0 0 96.4 Example center and Middle 0 0 0 95.2 of smallat Lower 0 0 0 95.7 this outer invention edge Small at Upper 41 3.4 ×10⁴ 5.0 × 10³ 70.3 Compara- center and Middle 38 2.8 × 10⁴ 6.2 × 10³72.1 tive large at Lower 40 3.1 × 10⁴ 5.3 × 10³ 71.5 example outer edge

As described in the foregoing, according to the fourth high-qualitysilicon single crystal and the method of producing the same, ahigh-quality single crystal of a large diameter and a long size in whichgrown-in defects such as dislocation clusters and laser scatteringtomography defects are minimized can be produced by a CZ method at ahigher pulling rate, i.e., at a higher level of productivity. Wafersproduced from the thus obtained single crystal contain less harmfuldefects which deteriorate device characteristics and hence can beeffectively adapted to larger scale integration and size reduction ofthe devices.

5. Fifth High-Quality Silicon Single Crystal and Method of Producing theSame

When a silicon single crystal is grown from a melt by CZ method, in viewof the diffusing behavior of vacancies and interstitial Si atomsintroduced into the crystal from the solid-melt interface, the followinghypothesis can be established. Laser scattering tomography defects wouldbe formed if the crystal is cooled with excessive vacancies, anddislocation clusters would result if the crystal is cooled withexcessive interstitial Si atoms. Further, if the crystal is cooled withvacancies and interstitial Si atoms balancing in number, both types ofdefects would disappear and R-OSF and an oxygen precipitation-promotingregion would be formed adjacent to where these types of defects wouldhave disappeared. From this hypothesis, the following can be inferred.As disclosed in the previously described Japanese Unexamined ApplicationLaid-Open No. 8-330316(1996), what would be required is that the averagetemperature gradient in the direction of the pulling shaft within asingle crystal immediately after solidification be almost the same bothat the center and at the outer edge or become gradually smaller from thecenter to the outer edge. However, the same Japanese UnexaminedApplication Laid-Open No. 8-330316(1996) does not refer to any specificmeans for achieving such a temperature distribution within the singlecrystal during pulling.

In inventing a fifth high-quality silicon single crystal and a method ofproducing the same, the inventors first investigated the possibility ofdisposing a heat-shielding body or the like for cooling orheat-insulation around the crystal being grown in order to change thetemperature gradients in the direction of the pulling shaft within thesilicon single crystal immediately after solidification. However, inview of problems, such as contamination and interference with operation,encountered when a foreign object is moved closer to the melt surface,the above attempt was not so effective as expected. As a next attempt,examinations were made as to how effective it would be to change therotating speed of the single crystal and that of the crucible duringpulling which is the technique employed in the normal single crystalgrowth. It was found out from the examination results that a singlecrystal capable of supplying wafers containing an extremely small levelof laser scattering tomography defects and dislocation dusters can beproduced by controlling the rotating speed of the crucible or of thesingle crystal or of both and by limiting the pulling rate.

FIG. 28 schematically shows the solid-melt interfaces of single crystalsthat are being pulled. The single crystal that is being pulled releasesmuch of latent heat of solidification from its surface, and thus itssurface is cooled faster than its inside. Hence, the surface temperatureis higher than the inside temperature, and the solidifying interface orthe solid-melt interface usually tends to be upwardly convex with thetemperature higher at the center of the single crystal than at thesurface. The temperature distribution within the crystal is such thatthe temperature is higher at the center on a plane perpendicular to thepulling shaft, i.e., as schematically shown in FIG. 28(a). Here, thetemperature on the solid-melt interface is maintained at a predeterminedvalue, which is the solidifying point of silicon. Now, assuming thedistances from the solid-melt interface to an isotherm having the sametemperature difference (ΔT) in the direction of the pulling shaft withinthe single crystal, a distance (Lc) at the center is greater than adistance (Ls) at the surface. That is, the temperature gradient in thedirection of the pulling shaft at the center of the single crystal Gc(=ΔT/Lc) is smaller than the temperature gradient in the same directionat the surface Gs (=ΔT/Ls).

In contrast, supposing that the single crystal would be cooled under thesame conditions immediately after pulling, if the solid-melt interfaceis made further upwardly convex than isotherms within the single crystalas shown in FIG. 28(b), then Ls becomes greater than Lc, and thus Gcmust be made greater than Gs. If Gc≧Gs can be satisfied immediatelyafter solidification, then it would be possible, as will be describedlater, to expand the angle of the V-shaped distribution of R-OSF that isso shaped in function of the pulling rate as previously described withreference to FIG. 2. Thus, methods of achieving this state were studied.

The solid-melt interface tends to be further upwardly convex withincreasing single crystal pulling rate. This is because the faster thepulling rate, the slower the release of latent heat of solidification atthe center of the single crystal than at the surface, and thus thetemperature difference increases between the center and the outer edge.However, an upwardly convex solid-melt interface can be obtained byincreasing the pulling rate, but this is not enough in that the numberof laser scattering tomography defects is increased. On the other hand,when the pulling rate is decreased, the solid-melt interface becomesflat or downwardly convex. This is because lower pulling rates allow thecrystal to release its latent heat of solidification sufficiently,thereby preventing heat from resting at the center, and thus Ls tends toincrease. However, a mere reduction in pulling rate is not enoughbecause dislocation clusters begin to occur.

During single crystal growth, the crucible is rotated, generally, atabout 5-15 rpm and the single crystal at about 15-30 rpm in order to,e.g., achieve a temperature distribution symmetrical with respect to thecenter over the solid-melt interface, reduce erratic temperaturefluctuations due to heat convection, and homogenize impurities andaddition elements. Since the melt within the crucible is heated by aheater from its outer edge, upward natural convection occurs near theside wall of the crucible and downward convection at the center. Whenthe crucible is rotated, such movements of the melt within the crucibleare restrained.

However, when the crucible is rotated faster, an upwardly convexsolid-melt interface is harder to obtain, and thus it was found out thatit was desirable to rotate the crucible as slowly as possible. When thesingle crystal is rotated, forced convection, or a cock-run flowresults. This forced convection is an upward flow occurring at thecenter of the crucible, and a relatively high-temperature melt hits themiddle of the single crystal and flows from the middle to the outeredge, thereby increasing the temperature of the solid-melt interface atthe center and hence making the solid-melt interface being furtherupwardly convex.

Thus, by making the solid-melt interface flat or upwardly convex whilecombining the rotating speed of the crucible and that of the singlecrystal in such a pulling rate range that the outside diameter of R-OSFis sufficiently small, a temperature distribution such as shown in FIG.28(b) is achieved. It has been verified that by such a method, a singlecrystal capable of supplying wafers having an extremely small level ofgrown-in defects such as laser scattering tomography defects anddislocation clusters can be produced. By further defining the producingcondition limits, the method of producing the fifth high-quality siliconsingle crystal has been accomplished.

The method is characterized by pulling a single crystal in such a statethat the shape of a solid-melt interface between the single crystalbeing grown and a melt is flat or upwardly convex at such a low ratethat the outside diameter of R-OSF occurring in the crystal is within arange of 0-60% of the diameter of the crystal. In this producing method,it is desirable that the rotating speed of the crucible be 5 rpm orless, or/and the rotating speed of the single crystal be 13 rpm or more.

Now, let us think about why a denuded zone is expanded by selecting anappropriate pulling rate when the temperature gradients in the directionof the pulling shaft within a single crystal immediately aftersolidification are substantially the same at the center and at the outeredge of the crystal, or become gradually smaller from the center to theouter edge. First, the melt undergoes a change to a solid crystalthrough solidification when pulled at the time of single crystal growth.Such a change takes place from a liquid phase in which atoms arerandomly arranged to a solid phase in which atoms are neatly arranged,and thus in the solid phase near the solid-melt interface, there arevacancies created by atoms that should exist missing and interstitialatoms created by superfluous Si atoms entering a crystal lattice inlarge amounts. It is presumed that there would be more vacancies createdby missing atoms than interstitial atoms in the crystal immediatelyafter solidification. As the portion grown into a single crystal aftersolidified by pulling moves away from the solid-melt interface, atomsand vacancies move and diffuse or vacancies and interstitial atoms arecombined together, etc., so that these vacancies and interstitial atomsdisappear, causing atoms in the crystal to be neatly arranged. However,due to the decreased moving and diffusing rates caused by temperaturedrops, some of these vacancies and interstitial atoms do remain in thecrystal.

Of the vacancies and interstitial atoms introduced in the solidificationprocess, the former outnumbers the latter, and both can move aroundwithin the crystal quite freely while the crystal temperature is high.It is assumed that vacancies move or diffuse faster than interstitialatoms. Here, the saturation limit concentrations of vacancies andinterstitial atoms allowed to be present in a high-temperature crystalare smaller at lower temperatures, respectively. Thus, even if both ofthem are present in equal amounts, one at lower temperatures has higherconcentrations and the other at higher temperatures has lowerconcentrations as the actual effect. A single crystal being grown hastemperature gradients in the vertical direction, and actualconcentration differences attributable to these temperature gradientscause diffusion to occur against the temperature gradients from thelow-temperature side to the high-temperature side, i.e., from the upperside of the single crystal being grown to the solid-melt interface, andthus the number of vacancies and interstitial atoms decreases withdecreasing temperature. Since vacancies are created when atomsconstituting a crystal lattice are missing and interstitial atoms arecreated when superfluous atoms are present in the lattice, when both ofthem collide, they are combined with each other to disappear, and thusthe crystal lattice tends to become perfect.

Temperature gradients in the direction of the vertical axis of a crystalbeing grown little change even if the pulling rate is changed as long asthe design of a hot zone, i.e., a portion to be cooled of the singlecrystal being pulled is the same. Vacancies and interstitial atomsdiffuse and they combine and disappear actively in the temperature rangeof the solidifying point (1412° C.) to approximately 1250° C., and it isassumed that their combination and disappearance caused by theirdiffusion do proceed even at temperatures lower than the above althoughtheir diffusion rate is decreased. At the same temperature gradient inthe same temperature range, vacancies diffuse toward the solid-meltinterface against the temperature gradient in substantially the sameamount per unit time. Hence, when the pulling rate is increased, thecrystal temperature decreases, leaving undiffused those vacancies thatoutnumber interstitial atoms, and even if vacancies continue todisappear to some extent through their diffusion to the surface andtheir combination with interstitial atoms, the undiffused vacancies doremain in the crystal as defects. It is assumed that these undiffusedvacancies would be the cause of laser scattering tomography defects.This is equivalent to a portion in which the pulling rate is high inFIG. 2 described previously. On the other hand, when the pulling rate islow, which is equivalent to the lower portion in FIG. 2, vacanciesdiffuse and disappear sufficiently, but interstitial atoms, whichdiffuse more slowly than vacancies, are left undiffused while thecrystal temperature decreases with vacancies relatively running short,and thus these undiffused interstitial atoms that are superfluous in theend cause dislocation clusters. Hence, laser scattering tomographydefects mainly result in a rapidly grown single crystal portion wherethe pulling rate is high and dislocation clusters mainly result in aslowly grown single crystal portion where the pulling rate is low. Awafer sliced from a crystal portion located in between includes both ofthese crystal portions.

In the normal single crystal pulling and growing method, the temperaturegradient at the center Gc is larger than the temperature gradient at thesurface Gs as has been described with reference to FIG. 28(a). That is,the concentrations of vacancies and interstitial atoms drop faster atthe surface than at the center due to their temperature gradient-baseddiffusion. However, since vacancies diffuse very much faster thaninterstitial atoms, the distribution of vacancy concentrations becomesanalogous to isotherms within the crystal on a surface of a wafer thatis perpendicular to the pulling shaft, while concentrations ofinterstitial atoms are distributed almost uniformly on a plane that isperpendicular to the pulling shaft. Further, defects such as vacanciesand interstitial atoms disappear upon reaching the surface of a crystal,and thus their concentrations are low at the surface, so that theirtemperature gradient-based diffusion to the surface are also occurringin addition to their temperature gradient-based diffusion.

FIG. 29 illustrates the relationship between the temperature gradientsat the center and at the surface in a single crystal being pulled andthe densities of vacancies and interstitial atoms. In theabove-described normal single crystal pulling and growing method, thesame relationship is presumed to be as schematically shown in FIG.29(a). Even if there are differences in concentration between vacanciesand interstitial atoms, when the pulling rate is high, vacancies areexcessive and thus results a rapidly grown single crystal in which laserscattering tomography defects tend to occur in the whole in-plane area,while when the pulling rate is low, interstitial atoms are excessive andthus results a slowly grown single crystal in which dislocation clusterstend to occur in the whole in-plane area.

However, at intermediate pulling rates, the crystal temperaturedecreases with the vacancy concentration being close to the interstitialatom concentration, but their concentration distributions are differentdue to different temperature gradients in the direction of the pullingshaft and different diffusion rates, and thus, as indicated by themiddle portion in FIG. 29(a), vacancies exceed interstitial atoms at thecenter of the crystal, while vacancies run short at a portion near thesurface. That is, laser scattering tomography defects are mainlydistributed around the center, and dislocation clusters near the surfacearound the outer edge. In the middle portion between the surface and thecenter, vacancies balance with interstitial atoms in number, and thusboth are combined together to disappear. As a result, formed is adenuded zone free of any of these types of grown-in defects that occurin the rapidly grown single crystal and the slowly grown single crystal,with R-OSF occurring at substantially the same place. Nucleation ofoxygen precipitates accounts for the formation of R-OSF. The fact thatan oxygen precipitation-promoting region exists next to R-OSF seems tosupport such account. Although grown-in defects such as laser scatteringtomography defects and dislocation clusters are not present in R-OSF orin the oxygen precipitation-promoting region, when oxygen precipitatesare formed, vacancies and the like are assumed to form the nucleation ofthese precipitates. Thus, vacancies remaining in these regions in smallamounts would be lost through oxygen precipitation. Thus, it is presumedthat the fact that the distribution of vacancy concentrations exhibits alarger difference than that of interstitial atom concentrations acrossthe radius of a single crystal, i.e., between the center and the surfacewould contribute to bringing about such a V-shaped distribution of FIG.2 that the diameters of R-OSF and the denuded zone are reduced as thepulling rate is decreased.

As described in the foregoing, if the formation of a denuded zone isdependent on the fact that vacancies balance with interstitial atoms innumber, and if such balancing is governed by temperature gradients inthe vertical direction of a crystal in a high temperature rangeimmediately after solidification as described above, then it is assumedthat to expand the denuded zone, the pulling rate may be adjusted insuch a manner that temperature gradients in the direction of thevertical axis in a single crystal being pulled are the same in size in aplane perpendicular to the pulling shaft, i.e., in the in-plane area ofa wafer. However, vacancies and interstitial atoms diffuse toward thesurface of the crystal in addition to their temperature gradient-baseddiffusion, and thus when the temperature gradients in the direction ofthe pulling shaft are made the same both at the center and at thesurface, the vacancy concentration at the surface is reducedexcessively, so that it is desirable that the temperature gradient inthe direction of the pulling shaft at the surface be smaller than at thecenter.

In the fifth high-quality silicon single crystal, the shape of thesolid-melt interface is made flat of upwardly convex as a means forkeeping the temperature gradient in the direction of the pulling shaftequal both at the center of the crystal and at its surface or slightlysmaller at the surface, in addition to decreasing the pulling rate sothat R-OSF is positioned toward the center of a wafer. By doing so, thedistribution of vacancy concentrations is further flattened to be closeto that of interstitial atom concentrations as shown in FIG. 29(b), andby further selecting an appropriate pulling rate, a single crystal inwhich a denuded zone is expanded can be obtained.

In the method of producing the fifth high-quality silicon singlecrystal, it is required to select such a pulling rate that the outsidediameter of R-OSF observed in a wafer is within a range of 0-60% of thediameter of the crystal. During such a pulling operation, the methodalso requires that the average temperature gradient in the direction ofthe pulling shaft be equal at the center of the single crystal and atthe surface or be smaller at-the surface than at the center while thetemperature of the crystal being grown stays in the range of thesolidifying point to about 1250° C. As a result, a silicon singlecrystal having an extremely small level of grown-in defects such aslaser scattering tomography defects and dislocation clusters can beobtained. When the pulling rate is so decreased as required by themethod, the solid-melt interface usually tends to be flat or evendownwardly convex. However, in order to obtain the above-describedtemperature distribution within a single crystal, the shape of thesolid-melt interface of the single crystal that is being grown must beflat or upwardly convex.

In the method of producing the fifth high-quality silicon singlecrystal, the pulling rate at the time of single crystal growth is suchthat the outside diameter of R-OSF observed on a wafer is within a rangeof 0-60% of the diameter of the single crystal. While such outsidediameter of R-OSF changes in accordance with the pulling rate, thepulling rate for obtaining the same outside diameter of R-OSF differsdepending on the temperature conditions of a single crystal being pulledand the hot zone design for a single crystal being grown. In view ofthis, how the outside diameter of R-OSF changes is examinedexperimentally by changing the pulling rate using growing equipment, anda single crystal is grown at such a pulling rate that the outsidediameter falls within the above-described range.

When the pulling rate is so high that the outside diameter of R-OSFexceeds 60%, a region where laser scattering tomography defects occurremains around the center of a single crystal. Further, when the pullingrate is continuously decreased, the outside diameter of R-OSF isgradually reduced, finally, to 0%. If the pulling rate is furtherdecreased from such a level that the outside diameter of R-OSF is 0%,dislocation clusters begin to occur. To avoid the above inconvenience, asingle crystal is to be grown at such a pulling rate that the outsidediameter of R-OSF is within the range of 0-60% of its diameter. Thespecific pulling rate range differs depending on the structure of asingle crystal producing apparatus to be used, or the structure of a hotzone in particular. Therefore, it is desirable to select a proper valuethrough actually growing single crystals, taking wafers from the grownsingle crystals, and observing R-OSF in the wafers.

The single crystal is supposed to be pulled in such a state that theshape of the solid-melt interface is flat or upwardly convex. Althoughthe shape of the solid-melt interface cannot always be checked duringpulling, it can be checked by observing the single crystal after grown.For example, the crystal after completely pulled is split lengthwise,heat-treated at 800° C. for 4 hours and at 1000° C. for 16 hours whenits oxygen content is high, or heat-treated by heating it up to 900° C.at a rate of 5° C./min while charged into a furnace of about 650° C.,soaking it for 20 hours, thereafter heating it to 1000° C. at a rate of10° C./min, and soaking it at the same temperature for 10 hours when itsoxygen content is low, thereby causing precipitates of oxygen to beformed in the crystal. Thereafter, striations indicating the shape ofthe solid-melt interface can be observed through X-ray topography.

The reason why the shape of the solid-melt interface is made flat orupwardly convex is because by arranging so, the temperature gradient inthe direction of the pulling shaft can be smaller at the surface than atthe center of a single crystal immediately after solidification.Further, the surface of the single crystal immediately after pulled isheated by radiation from the melt surface and the heater, and hence thetemperature gradient in the vertical direction may become smaller at thesurface than at the center in some cases when the pulling rate isdecreased. In such cases, the solid-melt interface may be flat.

The crucible must be rotated during single crystal growth in order toobtain a single crystal of a predetermined shape through uniform heatingby the heater. However, its rotating speed is set at 5 rpm or less inthis invention. This is because, when the rotating speed of the crucibleis increased, it becomes difficult to implement ultra-low imperfectionin the whole in-plane area of a wafer. When the crucible is rotated, thefluidity of the melt within the crucible is restrained. Thus, when itsrotating speed exceeds 5 rpm, it is assumed that flows of melt such asto achieve a flat or upwardly convex solid-melt interface would bedisturbed.

To compare the effects of the rotating speed of the crucible,examinations were made as to how the distribution of defects changes bychanging the rotating speed of the crucible and by continuously changingthe pulling rate when an 8″-diameter single crystal was grown by melting120 kg of polysilicon, i.e., a raw material into which boron, a p-typedopant, was added so as to obtain an electrical resistivity of 10 Ωcm,using a single crystal producing apparatus.

FIG. 30 shows the results of the examinations made as to thedistributions of defects in the case where the rotating speed of thecrucible was varied at levels of 10 rpm, 3 rpm and 1 rpm. In the case ofFIG. 30, the single crystal was rotated at a constant speed of 20 rpm,and after forming the shoulder, the single crystal was grown to a lengthof about 50 mm at a pulling rate of 0.7 mm/min, after which the pullingrate was continuously decreased to 0.3 mm/min to grow the single crystalto a length of about 1000 mm. As to the thus obtained single crystal,FIG. 30 shows schematically the distributions of its defects in itsvertical cross section parallel with the pulling shaft. The defects wereobserved at the center of the obtained crystal. The distributions ofdefects in wafers when the pulling rate was varied can be inferred fromthese results.

In the case where the crucible was rotated at 10 rpm as shown in FIG.30(a), R-OSF moved from the outer edge to the center as the pulling ratewas decreased, so that laser scattering tomography defects that tend tobe easily formed in the inner side of R-OSF were reduced, butdislocation clusters, in turn, began to occur at the outer edge. Thatis, no matter how the pulling rate was varied, wafers free of grown-indefects, such as laser scattering tomography defects or dislocationclusters, could not be obtained. In contrast, in the case where thecrucible was rotated at 3 rpm such as shown in FIG. 30(b), wafers almostfree of grown-in defects could be obtained if the pulling rate wasdecreased to decrease the outside diameter of R-OSF. Further, when thecrucible was rotated at 1 rpm as shown in FIG. 30(c), a single crystalcapable of providing wafers free of grown-in defects could be producedin such a wide pulling rate range as to reduce the outside diameter ofR-OSF or eliminate R-OSF.

As described in the foregoing, the rotating speed of the crucible is setat 5 rpm or less as the upper limit, while its lower limit is notparticularly specified, and it may be 0 rpm.

A single crystal that is being pulled must be rotated at a rotatingspeed of 13 rpm or more. This is required to cause an upward flow offorced convection at the center of the crucible and a downward flow nearthe crucible wall. By this fluidity of the melt, the high-temperatureupward flow of the melt hits the central portion of the crucible, i.e.,the middle portion of the lower surface of the crystal being grown,thereby allowing the solid-melt interface to be maintained upwardlyconvex. When the single crystal is rotated at speeds below 13 rpm, thesingle crystal that can provide wafers that are free of defects in theirwhole in-plane area cannot be obtained. On the other hand, when thesingle crystal is rotated too fast, a wafer region having an extremelylow level of defects decreases, and the growing rate of the crystal alsodecreases. The reason for this is assumed to be that the upward flowpasses near the solid-melt interface so fast that the interface does notbecome upwardly convex satisfactorily. Therefore, it is desirable thatthe single crystal be rotated at 30 rpm at most. That is, the crystal isrotated at 13 rpm or more. Its desirable range is between 15 and 30 rpm.

The design of the cooled portion of a single crystal that is pulled fromthe melt, i.e., the design of a hot zone is not particularly limited.However, while the crystal stays in the temperature range of thesolidifying point to about 1250° C., it is desirable that thetemperature gradient in the direction of the pulling shaft is not largeat the surface of the crystal, and thus it is preferable that thesurface of the single crystal immediately above the melt surface be ofsuch a structure that radiation from the crucible wall or the heater isnot particularly shielded.

5-1. EXAMPLE 15

An 8″-diameter silicon single crystal was grown while changing therotating speed of the crystal and that of the crucible using the singlecrystal producing apparatus shown in FIG. 1. The crucible was chargedwith 120 kg of polysilicon that is a raw material, into which boron, ap-type dopant, was added so that the crystal has an electricalresistivity of about 10 Ωcm.

FIG. 31 shows the results of examinations made as to Example 15. Itshows the measurements of the pulling rate and the rotating speeds ofthe crystal and the crucible at which the single crystal was grown. TheR-OSF position was observed through X-ray topography as to wafers thatwere taken at the upper, middle, and lower portions of the singlecrystal, immersed into a 16%-by-weight aqueous solution of coppernitrate to have Cu deposited thereon, and heated at 900° C. for 20minutes and thereafter cooled. Further, the density of laser scatteringtomography defects and the density of dislocation clusters were examinedthrough laser scattering tomograpy and Secco etching, respectively.Further, as to wafers taken at positions adjacent to those wafers whosedefect distributions were examined, time-zero dielectric breakdown(TZDB) for an oxide film thickness of 25 nm was measured and theirpercent nondefective was obtained after subjecting each of such wafersto a predetermined heat treatment and the like and thereafter giving ita gate structure of a device.

The results of these examinations are collectively indicated in FIG. 31.The density of laser scattering tomography defects and that ofdislocation clusters were indicated in terms of the average of themeasurements at five arbitrary positions of each wafer. As is apparentfrom these results, the wafers obtained from the single crystal that wasgrown in accordance with the method specified by this invention were ofhigh quality with less grown-in defects such as laser scatteringtomography defects and dislocation clusters and with a higher percentnondefective in terms of TZDB compared with the wafers obtained from thesingle crystal that was grown by the conventional producing method.

As described in the foregoing, according to the fifth high-qualitysilicon single crystal and the method of producing the same, ahigh-quality single crystal of a large diameter and a long size in whichgrown-in defects such as dislocation clusters and laser scatteringtomography defects are minimized can be produced by CZ method in a goodyield. Wafers obtained from the thus produced single crystal containless harmful defects which deteriorate device characteristics and hencecan be effectively adapted to larger scale integration and sizereduction of the devices.

Industrial Applicability

In the high-quality silicon single crystals and the methods of producingthe same of this invention, the position where R-OSF occur can becontrolled in accordance with the single crystal pulling conditions, andat the same time, a high-quality single crystal of a large diameter anda long size in which grown-in defects such as dislocation clusters andlaser scattering tomography defects are minimized can be produced in agood yield. Wafers obtained from the thus produced single crystals aredenuded of harmful defects that deteriorate device characteristics andhence can be effectively adapted to larger scale integration and sizereduction of the devices.

Therefore, the high-quality silicon single crystals and the methods ofproducing the same according to this invention can be utilized in thefield of producing silicon single crystals for the preparation ofsemiconductor.

What is claimed is:
 1. A silicon single crystal grown by a Czochralskimethod, characterized in that the width of ring-like extendingoxidation-induced stacking faults exceeds 8% of the radius of said growncrystal and dislocation clusters are absent.
 2. A silicon single crystalgrown by a Czochralski method, characterized in that the width ofringlike extending oxidation-induced stacking faults exceeds 8% of theradius of said grown crystal, the inside diameter of said ring-likeextending oxidation-induced stacking faults is within a range of 0-80%of the diameter of said grown crystal, and grown-in defects are absent.3. A silicon single crystal grown by a Czochralski method, characterizedin that the outside diameter of a region where ring-like extendingoxidation-induced stacking faults occur are within a range of 0-80% andwithin a range of 0-33% of the diameter of said grown crystal,respectively, and dislocation clusters are absent.
 4. A silicon singlecrystal grown by a Czochralski method, characterized in that the insidediameter of a ring-like oxygen precipitation promoting region is withina range of 0-80% of the diameter of said grown crystal, the insidediameter of a region where ring-like extending oxidation-inducedstacking faults occur which is the inner side of said oxygenprecipitation promoting region is within a range of 0-33% of thediameter of said grown crystal, and dislocation clusters are absent. 5.A silicon single crystal grown-under such a condition that said crystalstays in a temperature range of 1250° C.-1000° C. for 7 hours or morewhen pulled by a Czochralski method, characterized in that the outsidediameter of ringlike extending oxidation-induced stacking faults iswithin a range of 0-60% of the diameter of said grown crystal.
 6. Asilicon single crystal grown under such a condition that said crystalstays in a temperature range of 1250° C.-1000° C. for 7 hours or morewhen pulled by a Czochralski method, characterized in that the insidediameter or the outside diameter of an oxygen precipitation promotingregion is within a range of 0-60% of the diameter of said grown crystal.7. A silicon single crystal grown under such a condition that saidcrystal stays in a temperature range of 1250° C.-1000° C. for 7 hours ormore when pulled by a Czochralski method, characterized in that theoutside diameter of a circular region where laser scattering tomographydefects are detected is within a range of 0-60% of the diameter of saidgrown crystal.
 8. A method of producing a silicon single crystalcharacterized in that said single crystal is grown under such conditionsthat said crystal is pulled so as to stay in a temperature range of1250° C.-1000° C. for 7 hours or more, and that the outside diameter ofring-like extending oxidation-induced stacking faults is within a rangeof 0-60% of the diameter of said grown crystal.
 9. A method of producinga silicon single crystal characterized in that said single crystal isgrown under such conditions that said crystal is pulled so as to stay ina temperature range of 1250° C.-1000° C. for 7 hours or more, and thatthe inside diameter or the outside diameter of an oxygen precipitationpromoting region is within a range of 0-60% of the diameter of saidgrown crystal.
 10. A method of producing a silicon single crystalcharacterized in that said single crystal is grown under such conditionsthat said crystal is pulled so as to stay in a temperature range of1250° C.-1000° C. for 7 hours or more, and that the outside diameter ofa circular region where laser scattering tomography defects are detectedis within a range of 0-60% of the diameter of said grown crystal.
 11. Amethod of producing a silicon single crystal characterized in that saidcrystal is pulled in such a state that the shape of a solid-meltinterface between said single crystal being grown and a melt is upwardlyconvex at such a low rate as to allow the outside diameter of ring-likeextending oxidation-induced stacking faults within said single crystalto be within a range of 0-60% of the diameter of said crystal, whereinthe rotating speed of a crucible is set at 5 rpm or less, and therotating speed of said single crystal is set at 13 rpm or more.